We are grateful to the Environmental Services Association Education Trust, the Synthetic Biology in Cambridge SynBio fund and the Ministry of Social Justice and Empowerment, Government of India, for financial support.

<ol>
	<li>Zwirglmaier, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Global+phylogeography+of+marine+Synechococcus+and+Prochlorococcus+reveals+a+distinct+partitioning+of+lineages+among+oceanic+biomes.">Global phylogeography of marine Synechococcus and Prochlorococcus reveals a distinct partitioning of lineages among oceanic biomes.</a> Environ Microbiol. 10, 147-161 (2008).</li><li>Galloway, J. N., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nitrogen+cycles:+past,+present,+and+future.">Nitrogen cycles: past, present, and future.</a> Biogeochemistry. 70, 153-226 (2004).</li><li>Lea-Smith, D. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Contribution+of+cyanobacterial+alkane+production+to+the+ocean+hydrocarbon+cycle.">Contribution of cyanobacterial alkane production to the ocean hydrocarbon cycle.</a> Proc Natl Acad Sci U S A. , (2015).</li><li>Howe, C. J., Barbrook, A. C., Nisbet, R. E. R., Lockhart, P. J., Larkum, A. W. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+origin+of+plastids.">The origin of plastids.</a> Philos Trans R Soc Lond B Biol Sci. 363, 2675-2685 (2008).</li><li>Lea-Smith, D. J., Bombelli, P., Vasudevan, R., Howe, C. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Photosynthetic,+respiratory+and+extracellular+electron+transport+pathways+in+cyanobacteria.">Photosynthetic, respiratory and extracellular electron transport pathways in cyanobacteria.</a> Biochim Biophys Acta. , (2015).</li><li>McCormick, A. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hydrogen+production+through+oxygenic+photosynthesis+using+the+cyanobacterium+Synechocystis+sp+PCC+6803+in+a+bio-photoelectrolysis+cell+(BPE)+system.">Hydrogen production through oxygenic photosynthesis using the cyanobacterium Synechocystis sp PCC 6803 in a bio-photoelectrolysis cell (BPE) system.</a> Energy Environ. Sci. 6, 2682-2690 (2013).</li><li>Bradley, R. W., Bombelli, P., Lea-Smith, D. J., Howe, C. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Terminal+oxidase+mutants+of+the+cyanobacterium+Synechocystis+sp.+PCC+6803+show+increased+electrogenic+activity+in+biological+photo-voltaic+systems.">Terminal oxidase mutants of the cyanobacterium Synechocystis sp. PCC 6803 show increased electrogenic activity in biological photo-voltaic systems.</a> Phys Chem Chem Phys. 15, 13611-13618 (2013).</li><li>Ducat, D. C., Way, J. C., Silver, P. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Engineering+cyanobacteria+to+generate+high-value+products.">Engineering cyanobacteria to generate high-value products.</a> Trends Biotechnol. 29, 95-103 (2011).</li><li>Dismukes, G. C., Carrieri, D., Bennette, N., Ananyev, G. M., Posewitz, M. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Aquatic+phototrophs:+efficient+alternatives+to+land-based+crops+for+biofuels.">Aquatic phototrophs: efficient alternatives to land-based crops for biofuels.</a> Curr Opin Biotechnol. 19, 235-240 (2008).</li><li>Tan, L. T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Bioactive+natural+products+from+marine+cyanobacteria+for+drug+discovery.">Bioactive natural products from marine cyanobacteria for drug discovery.</a> Phytochemistry. 68, 954-979 (2007).</li><li>Volk, R. B., Furkert, F. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Antialgal,+antibacterial+and+antifungal+activity+of+two+metabolites+produced+and+excreted+by+cyanobacteria+during+growth.">Antialgal, antibacterial and antifungal activity of two metabolites produced and excreted by cyanobacteria during growth.</a> Microbiol Res. 161, 180-186 (2006).</li><li>Scott, S. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Biodiesel+from+algae:+challenges+and+prospects.">Biodiesel from algae: challenges and prospects.</a> Curr Opin Biotechnol. 21, 277-286 (2010).</li><li>Lea-Smith, D. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Phycobilisome-deficient+strains+of+Synechocystis+sp.+PCC+6803+have+reduced+size+and+require+carbon-limiting+conditions+to+exhibit+enhanced+productivity.">Phycobilisome-deficient strains of Synechocystis sp. PCC 6803 have reduced size and require carbon-limiting conditions to exhibit enhanced productivity.</a> Plant Physiol. 165, 705-714 (2014).</li><li>Lea-Smith, D. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thylakoid+terminal+oxidases+are+essential+for+the+cyanobacterium+Synechocystis+sp.+PCC+6803+to+survive+rapidly+changing+light+intensities.">Thylakoid terminal oxidases are essential for the cyanobacterium Synechocystis sp. PCC 6803 to survive rapidly changing light intensities.</a> Plant Physiol. 162, 484-495 (2013).</li><li>Liu, X., Sheng, J., Curtiss, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fatty+acid+production+in+genetically+modified+cyanobacteria.">Fatty acid production in genetically modified cyanobacteria.</a> Proc Natl Acad Sci U S A. 108, 6899-6904 (2011).</li><li>Xu, H., Vavilin, D., Funk, C., Vermaas, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Multiple+deletions+of+small+cab-like+proteins+in+the+cyanobacterium+Synechocystis+sp+PCC+6803+-+Consequences+for+pigment+biosynthesis+and+accumulation.">Multiple deletions of small cab-like proteins in the cyanobacterium Synechocystis sp PCC 6803 - Consequences for pigment biosynthesis and accumulation.</a> J Biol Chem. 279, 27971-27979 (2004).</li><li>Castenholz, R. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Culturing+methods+for+Cyanobacteria.">Culturing methods for Cyanobacteria.</a> Method Enzymol. 167, 68-93 (1988).</li><li>Mitschke, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+experimentally+anchored+map+of+transcriptional+start+sites+in+the+model+cyanobacterium+Synechocystis+sp+PCC6803.">An experimentally anchored map of transcriptional start sites in the model cyanobacterium Synechocystis sp PCC6803.</a> Proc Natl Acad Sci U S A. 108, 2124-2129 (2011).</li><li>Ried, J. L., Collmer, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+nptI-sacB-sacR+cartridge+for+constructing+directed,+unmarked+mutations+in+gram-negative+bacteria+by+marker+exchange-eviction+mutagenesis.">An nptI-sacB-sacR cartridge for constructing directed, unmarked mutations in gram-negative bacteria by marker exchange-eviction mutagenesis.</a> Gene. 57, 239-246 (1987).</li><li>Vieira, J., Messing, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+pUC+plasmids,+an+M13mp7-derived+system+for+insertion+mutagenesis+and+sequencing+with+synthetic+universal+primers.">The pUC plasmids, an M13mp7-derived system for insertion mutagenesis and sequencing with synthetic universal primers.</a> Gene. 19, 259-268 (1982).</li><li>Vermaas, W. F. J., Williams, J. G. K., Rutherford, A. W., Mathis, P., Arntzen, C. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Genetically+Engineered+Mutant+of+the+Cyanobacterium+Synechocystis+6803+Lacks+the+Photosystem-Ii+Chlorophyll-Binding+Protein+Cp-47.">Genetically Engineered Mutant of the Cyanobacterium Synechocystis 6803 Lacks the Photosystem-Ii Chlorophyll-Binding Protein Cp-47.</a> Proc Natl Acad Sci U S A. 83, 9474-9477 (1986).</li><li>Westphal, S., Heins, L., Soll, J., Vothknecht, U. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Vipp1+deletion+mutant+of+Synechocystis:+A+connection+between+bacterial+phage+shock+and+thylakoid+biogenesis?.">Vipp1 deletion mutant of Synechocystis: A connection between bacterial phage shock and thylakoid biogenesis?.</a> Proc Natl Acad Sci U S A. 98, 4243-4248 (2001).</li><li>Zhang, S. Y., Shen, G. Z., Li, Z. K., Golbeck, J. H., Bryant, D. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Vipp1+Is+Essential+for+the+Biogenesis+of+Photosystem+I+but+Not+Thylakoid+Membranes+in+Synechococcus+sp+PCC+7002.">Vipp1 Is Essential for the Biogenesis of Photosystem I but Not Thylakoid Membranes in Synechococcus sp PCC 7002.</a> J Biol Chem. 289, 15904-15914 (2014).</li><li>Taroncher-Oldenberg, G., Nishina, K., Stephanopoulos, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Identification+and+analysis+of+the+polyhydroxyalkanoate-specific+beta-ketothiolase+and+acetoacetyl+coenzyme+A+reductase+genes+in+the+cyanobacterium+Synechocystis+sp+strain+PCC6803.">Identification and analysis of the polyhydroxyalkanoate-specific beta-ketothiolase and acetoacetyl coenzyme A reductase genes in the cyanobacterium Synechocystis sp strain PCC6803.</a> Appl Environ Microbiol. 66, 4440-4448 (2000).</li><li>Hein, S., Tran, H., Steinbuchel, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Synechocystis+sp.+PCC6803+possesses+a+two-component+polyhydroxyalkanoic+acid+synthase+similar+to+that+of+anoxygenic+purple+sulfur+bacteria.">Synechocystis sp. PCC6803 possesses a two-component polyhydroxyalkanoic acid synthase similar to that of anoxygenic purple sulfur bacteria.</a> Arch Microbiol. 170, 162-170 (1998).</li><li>Ng, A. H., Berla, B. M., Pakrasi, H. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fine+tuning+of+photoautotrophic+protein+production+by+combining+promoters+and+neutral+sites+in+Synechocystis+6803,+a+cyanobacterium.">Fine tuning of photoautotrophic protein production by combining promoters and neutral sites in Synechocystis 6803, a cyanobacterium.</a> Appl Environ Microbiol. , (2015).</li></ol>

The authors declare that they have no competing financial interests.

The most critical steps in generation of unmarked mutants are: 1) careful plasmid design to ensure only the targeted region is altered; 2) ensuring that samples remain axenic, especially when cultured on sucrose; 3) plating transformed cells for marked mutant generation initially on BG11 agar plates lacking antibiotics, followed by addition of agar plus antibiotics 24 hr later; 4) culturing marked mutants for 4 full days prior to plating on BG11 plus sucrose agar plates: 5) ensuring that marked mutants are fully segregat...

Cyanobacteria are an evolutionarily ancient and diverse phylum of bacteria found in nearly every natural environment on Earth. In marine ecosystems they are particularly abundant and play a key role in many nutrient cycles, accounting for approximately half of carbon fixation1, the majority of nitrogen fixation2 and hundreds of millions of tons of hydrocarbon production3 in the oceans annually. Chloroplasts, the organelle responsible for photosynthesis in eukaryotic algae and plants, are likely to have evolved from a cyanobacterium that was engulfed by a host organism4. Cyanobacteria have proved useful model organisms for the study of photosynthesis, electron transport5 and biochemical pathways, many of which are conserved in plants. In addition cyanobacteria are increasingly being used for production of food, biofuels6, electricity7 and industrial compounds8, due to their highly efficient conversion of water and CO2 to biomass using solar energy9. Many species can be cultivated on non-arable land with minimal nutrients and seawater, suggesting that cyanobacteria could potentially be grown at large scale without affecting agricultural production. Certain species are also sources of natural products, including antifungal, antibacterial and anti-cancer compounds10,11.
The ability to generate mutants is key to understanding cyanobacterial photosynthesis, biochemistry and physiology, and essential for development of strains for industrial purposes. The majority of published studies generate genetically modified strains by insertion of an antibiotic resistance cassette into the site of interest. This limits the number of mutations that can be introduced into a strain, as only a few antibiotic resistance cassettes are available for use in cyanobacteria. Strains containing genes conferring antibiotic resistance cannot be used for industrial production in open ponds, which is likely to be the only cost-effective means to produce biofuels and other low value products12. The generation of unmarked mutants overcomes these limitations. Unmarked mutants contain no foreign DNA, unless intentionally included, and can be manipulated multiple times. Therefore it is possible to generate as many alterations in a strain as desired. In addition, polar effects on genes downstream of the modification site can be minimized, allowing more precise modification of the organism13.
To generate mutant strains, suicide plasmids containing two DNA fragments identical to regions in the cyanobacterial chromosome flanking the gene to be deleted (termed the 5&#39; and 3&#39; flanking regions) are first constructed. Two genes are then inserted between these flanking regions. One of these encodes an antibiotic resistance protein; the second encodes SacB, which produces levansucrase, a compound conferring sensitivity to sucrose. In the first stage of the process, marked mutants, i.e. strains containing some foreign DNA, are generated. The plasmid construct is mixed with the cyanobacterial cells and the DNA is taken up naturally by the organism. Transformants are selected by growth on agar plates containing the appropriate antibiotic and the mutant genotype verified by PCR. Suicide plasmids cannot replicate within the strain of interest. Therefore any antibiotic resistant colonies will result from a recombination event whereby the gene of interest in inserted into the chromosome. To generate unmarked mutants, the marked mutant is then mixed with a second suicide plasmid containing just the 5&#39; and 3&#39; flanking regions. However, if insertion of foreign DNA is required, a plasmid consisting of the 5&#39; and 3&#39; flanking regions with a cassette containing the genes of interest inserted between these DNA fragments, can be used. Selection is via growth on agar plates containing sucrose. As sucrose is lethal to cells when the sacB gene product is expressed, the only cells that survive are those in which a second recombination event has occurred, whereby the sucrose sensitivity gene, in addition to the antibiotic resistance gene, has been recombined out of the chromosome and onto the plasmid. As a consequence of the recombinational exchange, the flanking regions and any DNA between them are inserted into the chromosome.
We have successfully used these methods to generate multiple chromosomal mutations in the same strain of Synechocystis sp. PCC6803 (hereafter referred to as Synechocystis)13,14, to introduce single point mutations into a gene of interest13 and for expression of gene cassettes. While generation of unmarked knockouts has been demonstrated prior to our work in Synechocystis15,16, a detailed method, aided by a visual presentation of the critical steps, is not publicly available. We have also applied the same method for generation of marked knockouts in another model cyanobacterium, Synechococcus sp. PCC7002 (hereafter referred to as Synechococcus). This protocol provides a clear, simple method for generating mutants and a rapid protocol for validating and storing these strains.

<table><tbody><tr><td>NaNO&lt;sub&gt;3&lt;/sub&gt;</td><td>Sigma</td><td>S5506</td><td></td></tr><tr><td>MgSO&lt;sub&gt;4&lt;/sub&gt;.7H&lt;sub&gt;2&lt;/sub&gt;O</td><td>Sigma</td><td>230391</td><td></td></tr><tr><td>CaCl&lt;sub&gt;2&lt;/sub&gt;</td><td>Sigma</td><td>C1016</td><td></td></tr><tr><td>citric acid</td><td>Sigma</td><td>C0759</td><td></td></tr><tr><td>Na&lt;sub&gt;2&lt;/sub&gt;EDTA</td><td>Fisher</td><td>EDT002</td><td></td></tr><tr><td>H&lt;sub&gt;3&lt;/sub&gt;BO&lt;sub&gt;3&lt;/sub&gt;</td><td>Sigma</td><td>339067</td><td></td></tr><tr><td>MnCl&lt;sub&gt;2&lt;/sub&gt;.4H&lt;sub&gt;2&lt;/sub&gt;O</td><td>Sigma</td><td>M3634</td><td></td></tr><tr><td>ZnSO&lt;sub&gt;4&lt;/sub&gt;.7H&lt;sub&gt;2&lt;/sub&gt;O</td><td>Sigma</td><td>Z4750</td><td></td></tr><tr><td>Na&lt;sub&gt;2&lt;/sub&gt;MoO&lt;sub&gt;4&lt;/sub&gt;.2H&lt;sub&gt;2&lt;/sub&gt;O</td><td>Sigma</td><td>331058</td><td></td></tr><tr><td>CuSO&lt;sub&gt;4&lt;/sub&gt;.5H&lt;sub&gt;2&lt;/sub&gt;O</td><td>Sigma</td><td>209198</td><td></td></tr><tr><td>Co(NO&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;.6H&lt;sub&gt;2&lt;/sub&gt;O</td><td>Sigma</td><td>239267</td><td></td></tr><tr><td>Ferric ammonium citrate</td><td>Sigma</td><td>F5879</td><td></td></tr><tr><td>K&lt;sub&gt;2&lt;/sub&gt;HPO&lt;sub&gt;4&lt;/sub&gt;</td><td>Sigma</td><td>P3786</td><td></td></tr><tr><td>Na&lt;sub&gt;2&lt;/sub&gt;CO&lt;sub&gt;3&lt;/sub&gt;</td><td>Fisher</td><td>SODC001</td><td></td></tr><tr><td>TES</td><td>Sigma</td><td>T1375</td><td></td></tr><tr><td>NaHCO&lt;sub&gt;3&lt;/sub&gt;</td><td>Fisher</td><td>SODH001</td><td></td></tr><tr><td>HEPES</td><td>Sigma</td><td>H3375</td><td></td></tr><tr><td>cyanocobalamin</td><td>Sigma</td><td>47869</td><td></td></tr><tr><td>Na&lt;sub&gt;2&lt;/sub&gt;S&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;</td><td>Sigma</td><td>72049</td><td></td></tr><tr><td>Bacto agar</td><td>BD</td><td>214010</td><td></td></tr><tr><td>Sucrose</td><td>Fisher</td><td>SUC001</td><td></td></tr><tr><td>Petri dish 90 mm triple vented</td><td>Greiner</td><td>633185</td><td></td></tr><tr><td>0.2 &amp;micro;m filters</td><td>Sartorius</td><td>16534</td><td></td></tr><tr><td>100 ml conical flasks</td><td>Pyrex</td><td>CON004</td><td></td></tr><tr><td>Parafilm M 100 mm x 38&amp;nbsp;m</td><td>Bemis</td><td>FIL003</td><td></td></tr><tr><td>Phusion high fidelity DNA polymerase&amp;nbsp;</td><td>Phusion</td><td>F-530</td><td></td></tr><tr><td>Agarose</td><td>Melford</td><td>MB1200</td><td></td></tr><tr><td>DNA purification kit&amp;nbsp;</td><td>MoBio</td><td>12100-300</td><td></td></tr><tr><td>Restriction endonucleases</td><td>NEB</td><td></td><td></td></tr><tr><td>T4 ligase</td><td>Thermo Scientific</td><td>EL0011</td><td></td></tr><tr><td>Luria Bertani broth</td><td>Invitrogen</td><td>12795-027</td><td></td></tr><tr><td>MES</td><td>Sigma</td><td>M8250</td><td></td></tr><tr><td>Kanamycin sulfate</td><td>Sigma</td><td>60615</td><td></td></tr><tr><td>Ampicillin</td><td>Sigma</td><td>A9518</td><td></td></tr><tr><td>GeneJET plasmid miniprep kit</td><td>Thermo Scientific</td><td>K0503</td><td></td></tr><tr><td>14 ml round-bottom tube</td><td>BD falcon</td><td>352059</td><td></td></tr><tr><td>GoTaq G2 Flexi DNA polymerase</td><td>Promega</td><td>M7805</td><td></td></tr><tr><td>425-600 &amp;micro;m glass beads</td><td>Sigma</td><td>G8772</td><td></td></tr><tr><td>Glycerol</td><td>Sigma</td><td>G5516</td><td></td></tr><tr><td>DMSO</td><td>Sigma</td><td>D8418</td><td></td></tr><tr><td>Fluorescent bulbs</td><td>Gro-Lux</td><td>69</td><td></td></tr><tr><td>HT multitron photobioreactor</td><td>Infors</td><td></td><td></td></tr></tbody></table>

generation of marked and markerless mutants in model cyanobacterial species

Cyanobacteria are ecologically important organisms and potential platforms for production of biofuels and useful industrial products. Genetic manipulation of cyanobacteria, especially model organisms such as Synechocystis sp. PCC6803 and Synechococcus sp. PCC7002, is a key tool for both basic and applied research. Generation of unmarked mutants, whereby chromosomal alterations are introduced into a strain via insertion of an antibiotic resistance cassette (a manipulatable fragment of DNA containing one or more genes), followed by subsequent removal of this cassette using a negative selectable marker, is a particularly powerful technique. Unmarked mutants can be repeatedly genetically manipulated, allowing as many alterations to be introduced into a strain as desired. In addition, the absence of genes encoding antibiotic resistance proteins in the mutated strain is desirable, as it avoids the possibility of 'escape' of antibiotic resistant organisms into the environment. However, detailed methods for repeated rounds of genetic manipulation of cyanobacteria are not well described in the scientific literature. Here we provide a comprehensive description of this technique, which we have successfully used to generate mutants with multiple deletions, single point mutations within a gene of interest and insertion of novel gene cassettes.

Plasmid design is critical for successful generation of both marked and unmarked mutants. Figure 1 gives an example of plasmid A and B used to generate a deletion mutant in the Synechocystis genes cpcC1 and cpcC213. In each case the 5&#39; and 3&#39; flanking regions are approximately 900-1,000 bp. Reduced flanking regions can be used although the smallest we have successfully trialed has been approximately 500 bp. Plasmid B can also ...

Watch this Scientific Journal Video about Generation of Marked and Markerless Mutants in Model Cyanobacterial Species at JoVE.com

Generation of Marked and Markerless Mutants in Model Cyanobacterial Species

Introducing multiple genomic alterations into cyanobacteria is an essential tool in the development of strains for industrial and basic research purposes. We describe a system for generating unmarked mutants in the model cyanobacterial species Synechocystis sp. PCC6803 and marked mutants in Synechococcus sp. PCC7002.

Cyanobacteria are ecologically important organisms and potential platforms for production of biofuels and useful industrial products. Genetic manipulation ...

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Biology

12.0K Views.  University of Cambridge. Introducing multiple genomic alterations into cyanobacteria is an essential tool in the development of strains for industrial and basic research purposes. We describe a system for generating unmarked mutants in the model cyanobacterial species Synechocystis sp. PCC6803 and marked mutants in Synechococcus sp. PCC7002.

Video: Generation of Marked and Markerless Mutants in Model Cyanobacterial Species

Optogenetic Random Mutagenesis Using Histone-miniSOG in C. elegans

Forward genetic screening in model organisms is the workhorse to discover functionally important genes and pathways in many biological processes. In most mutagenesis-based screens, researchers have relied on the use of toxic chemicals, carcinogens, or irradiation, which requires designated equipment, safety setup, and/or disposal of hazardous materials. We have developed a simple approach to induce heritable mutations in C. elegans using germline-expressed histone-miniSOG, a light-inducible potent generator of reactive oxygen species. This mutagenesis method is free of toxic chemicals and requires minimal laboratory safety and waste management. The induced DNA modifications include single-nucleotide changes and small deletions, and complement those caused by classical chemical mutagenesis. This methodology can also be used to induce integration of extrachromosomal transgenes. Here, we provide the details of the LED setup and protocols for standard mutagenesis and transgene integration.

Optogenetic Random Mutagenesis Using Histone-miniSOG in C. elegans

Genetically-encoded histone-miniSOG induces genome-wide heritable mutations in a blue&#160;light-dependent manner. This mutagenesis method is simple, fast, free of toxic chemicals, and well-suited for forward genetic screening and transgene integration.

Forward genetic screening in model organisms is the workhorse to discover functionally important genes and pathways in many biological processes. In most ...

Genetics

High-Throughput Robotically Assisted Isolation of Temperature-sensitive Lethal Mutants in Chlamydomonas reinhardtii

Systematic identification and characterization of genetic perturbations have proven useful to decipher gene function and cellular pathways. However, the conventional approaches of permanent gene deletion cannot be applied to essential genes. We have pioneered a unique collection of ~70 temperature-sensitive (ts) lethal mutants for studying cell cycle regulation in the unicellular green algae Chlamydomonas reinhardtii1. These mutations identify essential genes, and the ts alleles can be conditionally inactivated by temperature shift, providing valuable tools to identify and analyze essential functions. Mutant collections are much more valuable if they are close to comprehensive, since scattershot collections can miss important components. However, this requires the efficient collection of a large number of mutants, especially in a wide-target screen. Here, we describe a robotics-based pipeline for generating ts lethal mutants and analyzing their phenotype in Chlamydomonas. This technique can be applied to any microorganism that grows on agar. We have collected over 3000 ts mutants, probably including mutations in most or all cell-essential pathways, including about 200 new candidate cell cycle mutations. Subsequent molecular and cellular characterization of these mutants should provide new insights in plant cell biology; a comprehensive mutant collection is an essential prerequisite to ensure coverage of a broad range of biological pathways. These methods are integrated with downstream genetics and bioinformatics procedures for efficient mapping and identification of the causative mutations that are beyond the scope of this manuscript.

High-Throughput Robotically Assisted Isolation of Temperature-sensitive Lethal Mutants in Chlamydomonas reinhardtii

Temperature-sensitive (ts) lethal mutants are valuable tools to identify and analyze essential functions. Here we describe methods to generate and classify ts lethal mutants in high throughput.

Systematic identification and characterization of genetic perturbations have proven useful to decipher gene function and cellular pathways. However, the ...

Identifying Amino Acid Overproducers Using Rare-Codon-Rich Markers

To satisfy the ever-growing market for amino acids, high-performance production strains are needed. The amino acid overproducers are conventionally identified by harnessing the competitions between amino acids and their analogs. However, this analog-based method is of low accuracy, and proper analogs for specific amino acids are limited. Here, we present an alternative strategy that enables an accurate, sensitive, and high-throughput screening of amino acid overproducers using rare-codon-rich markers. This strategy is inspired by the phenomenon of codon usage bias in protein translation, for which codons are categorized into common or rare ones based on their frequencies of occurrence in the coding DNA. The translation of rare codons depends on their corresponding rare transfer RNAs (tRNAs), which cannot be fully charged by the cognate amino acids under starvation. Theoretically, the rare tRNAs can be charged if there is a surplus of the amino acids after charging the synonymous common isoacceptors. Therefore, retarded translations caused by rare codons could be restored by feeding or intracellular overproductions of the corresponding amino acids. Under this assumption, a selection or screening system for identifying amino acid overproducers is established by replacing the common codons of the targeted amino acids with their synonymous rare alternatives in the antibiotic resistance genes or the genes encoding fluorescent or chromogenic proteins. We show that the protein expressions can be greatly hindered by the incorporation of rare codons and that the levels of proteins correlate positively with the amino acid concentrations. Using this system, overproducers of multiple amino acids can be readily screened out from mutation libraries. This rare-codon-based strategy only requires a single modified gene, and the host is less likely to escape the selection than in other methods. It offers an alternative approach for obtaining amino acid overproducers.

This study presents an alternative strategy to the conventional toxic analog-based method in identifying amino acid overproducers by using rare-codon-rich markers to achieve accuracy, sensitivity, and high-throughput simultaneously.

To satisfy the ever-growing market for amino acids, high-performance production strains are needed. The amino acid overproducers are conventionally ...

Biochemistry

Visual and Microscopic Evaluation of Streptomyces Developmental Mutants

Streptomycetes are filamentous soil bacteria belonging to the phylum Actinobacteria that are found throughout the world and produce a wide array of antibiotics and other secondary metabolites. Streptomyces coelicolor is a well-characterized, non-pathogenic species that is amenable to a variety of analyses in the lab. The phenotyping methods described here use S. coelicolor as a model streptomycete; however, the methods are applicable to all members of this large genus as well as some closely related actinomycetes. Phenotyping is necessary to characterize new species of Streptomyces identified in the environment, and it is also a vital first step in characterizing newly isolated mutant strains of Streptomyces. Proficiency in phenotyping is important for the many new researchers who are entering the field of Streptomyces research, which includes the study of bacterial development, cell division, chromosome segregation, and second messenger signaling. The recent crowdsourcing of antibiotic discovery through the isolation of new soil microbes has resulted in an increased need for training in phenotyping for instructors new to the field of Streptomyces research and their college or high school students. This manuscript describes methods for bacterial strain propagation, storage, and characterization through visual and microscopic examination. After reading this article, new researchers (microbiology education laboratories and citizen scientists) should be able to manipulate Streptomyces strains and begin visual characterization experiments.

Visual and Microscopic Evaluation of Streptomyces Developmental Mutants

Here we present protocols for novice researchers to initiate phenotyping for the pharmacologically important bacterial genus Streptomyces.

Streptomycetes are filamentous soil bacteria belonging to the phylum Actinobacteria that are found throughout the world and produce a wide array of ...

Immunology and Infection

Live-Cell Forward Genetic Approach to Identify and Isolate Developmental Mutants in Chlamydia trachomatis

The intracellular bacterial pathogen Chlamydia trachomatis undergoes a developmental cycle consisting of two morphologically discrete developmental forms. The non-replicative elementary body (EB) initiates infection of the host. Once inside, the EB differentiates into the reticulate body (RB). The RB then undergoes multiple rounds of replication, before differentiating back to the infectious EB form. This cycle is essential for chlamydial survival as failure to switch between cell types prevents either host invasion or replication.
Limitations in genetic techniques due to the obligate intracellular nature of Chlamydia have hampered identification of the molecular mechanisms involved in the cell-type development. We designed a novel dual promoter-reporter plasmid system that, in conjunction with live-cell microscopy, allows for the visualization of cell type switching in real time. To identify genes involved in the regulation of cell-type development, the live-cell promoter-reporter system was leveraged for the development of a forward genetic approach by combining chemical mutagenesis of the dual reporter strain, imaging and tracking of Chlamydia with altered developmental kinetics, followed by clonal isolation of mutants. This forward genetic workflow is a flexible tool that can be modified for directed interrogation into a wide range of genetic pathways.

Live-Cell Forward Genetic Approach to Identify and Isolate Developmental Mutants in Chlamydia trachomatis

This protocol utilizes fluorescent promoter-reporters, live-cell microscopy, and individual inclusion extraction in a directed forward genetic approach to identify and isolate developmental mutants of Chlamydia trachomatis.

The intracellular bacterial pathogen Chlamydia trachomatis undergoes a developmental cycle consisting of two morphologically discrete developmental forms. ...

Stable Gene Integration in Chlorella vulgaris Using Agrobacterium tumefaciens

Agrobacterium tumefaciens-Mediated Genetic Engineering of Green Microalgae, Chlorella vulgaris

Agrobacterium tumefaciens-mediated transformation (AMT) serves as a widely employed tool for manipulating plant genomes. However, A. tumefaciens exhibit the capacity for gene transfer to a diverse array of species. Numerous microalgae species lack well-established methods for reliably integrating genes of interest into their nuclear genome. To harness the potential benefits of microalgal biotechnology, simple and efficient genome manipulation tools are crucial. Herein, an optimized AMT protocol is presented for the industrial microalgae species Chlorella vulgaris, utilizing the reporter green fluorescent protein (mGFP5) and the antibiotic resistance marker for Hygromycin B. Mutants are selected through plating on Tris-Acetate-Phosphate (TAP) media containing Hygromycin B and cefotaxime. Expression of mGFP5 is quantified via fluorescence after over ten generations of subculturing, indicating the stable transformation of the T-DNA cassette. This protocol allows for the reliable generation of multiple transgenic C. vulgaris colonies in under two weeks, employing the commercially available pCAMBIA1302 plant expression vector.

Author Spotlight: Optimized Transformation Protocol for Chlorella vulgaris Using Agrobacterium tumefaciens 

This protocol outlines the utilization of Agrobacterium tumefaciens-mediated transformation (AMT) for integrating gene(s) of interest into the nuclear genome of the green microalgae Chlorella vulgaris, leading to the production of stable transformants.

Agrobacterium tumefaciens-mediated transformation (AMT) serves as a widely employed tool for manipulating plant genomes. However, A. tumefaciens exhibit ...

Bioengineering

Generation of Null Mutants to Elucidate the Role of Bacterial Glycosyltransferases in Bacterial Motility

The study of glycosylation in prokaryotes is a rapidly growing area. Bacteria harbor different glycosylated structures on their surface whose glycans constitute a strain-specific barcode. The associated glycans show higher diversity in sugar composition and structure than those of eukaryotes and are important in bacterial-host recognition processes and interaction with the environment. In pathogenic bacteria, glycoproteins have been involved in different stages of the infectious process, and glycan modifications can interfere with specific functions of glycoproteins. However, despite the advances made in the understanding of glycan composition, structure, and biosynthesis pathways, understanding of the role of glycoproteins in pathogenicity or interaction with the environment remains very limited. Furthermore, in some bacteria, the enzymes required for protein glycosylation are shared with other polysaccharide biosynthetic pathways, such as lipopolysaccharide and capsule biosynthetic pathways. The functional importance of glycosylation has been elucidated in several bacteria through mutation of specific genes thought to be involved in the glycosylation process and the study of its impact on the expression of the target glycoprotein and the modifying glycan. Mesophilic Aeromonas have a single and O-glycosylated polar flagellum. Flagellar glycans show diversity in carbohydrate composition and chain length between Aeromonas strains. However, all strains analyzed to date show a pseudaminic acid derivative as the linking sugar that modifies serine or threonine residues. The pseudaminic acid derivative is required for polar flagella assembly, and its loss has an impact on adhesion, biofilm formation, and colonization. The protocol detailed in this article describes how the construction of null mutants can be used to understand the involvement of genes or genome regions containing putative glycosyltransferases in the biosynthesis of a flagellar glycan. This includes the potential to understand the function of the glycosyltransferases involved and the role of the glycan. This will be achieved by comparing the glycan deficient mutant to the wild-type strain.

Here, we describe the construction of null mutants of Aeromonas in specific glycosyltransferases or regions containing glycosyltransferases, the motility assays, and flagella purification performed to establish the involvement and function of their encoded enzymes in the biosynthesis of a glycan, as well as the role of this glycan in bacterial pathogenesis.

The study of glycosylation in prokaryotes is a rapidly growing area. Bacteria harbor different glycosylated structures on their surface whose glycans ...

Monitoring Intraspecies Competition in a Bacterial Cell Population by Cocultivation of Fluorescently Labelled Strains

Many microorganisms such as bacteria proliferate extremely fast and the populations may reach high cell densities. Small fractions of cells in a population always have accumulated mutations that are either detrimental or beneficial for the cell. If the fitness effect of a mutation provides the subpopulation with a strong selective growth advantage, the individuals of this subpopulation may rapidly outcompete and even completely eliminate their immediate fellows. Thus, small genetic changes and selection-driven accumulation of cells that have acquired beneficial mutations may lead to a complete shift of the genotype of a cell population. Here we present a procedure to monitor the rapid clonal expansion and elimination of beneficial and detrimental mutations, respectively, in a bacterial cell population over time by cocultivation of fluorescently labeled individuals of the Gram-positive model bacterium Bacillus subtilis. The method is easy to perform and very illustrative to display intraspecies competition among the individuals in a bacterial cell population.

Bacteria may accumulate either detrimental or beneficial mutations during their lifetime. In a population of cells individuals that have accumulated beneficial mutations may rapidly outcompete their fellows. Here we present a simple procedure to visualize intraspecies competition in a bacterial cell population over time using fluorescently labeled individuals.&#160;

Many microorganisms such as bacteria proliferate extremely fast and the populations may reach high cell densities. Small fractions of cells in a ...

Protocol for Co-Culturing Sucrose-Secreting Cyanobacteria with Fungal Partners

Assembly and Quantification of Co-Cultures Combining Heterotrophic Yeast with Phototrophic Sugar-Secreting Cyanobacteria

With the increasing demand for sustainable biotechnologies, mixed consortia containing a phototrophic microbe and heterotrophic partner species are being explored as a method for solar-driven bioproduction. One approach involves the use of CO2-fixing cyanobacteria that secrete organic carbon to support the metabolism of a co-cultivated heterotroph, which in turn transforms the carbon into higher-value goods or services. In this protocol, a technical description to assist the experimentalist in the establishment of a co-culture combining a sucrose-secreting cyanobacterial strain with a fungal partner(s), as represented by model yeast species, is provided. The protocol describes the key prerequisites for co-culture establishment: Defining the media composition, monitoring the growth characteristics of individual partners, and the analysis of mixed cultures with multiple species combined in the same growth vessel. Basic laboratory techniques for co-culture monitoring, including microscopy, cell counter, and single-cell flow cytometry, are summarized, and examples of nonproprietary software to use for data analysis of raw flow cytometry standard (FCS) files in line with FAIR (Findable, Accessible, Interoperable, Reusable) principles are provided. Finally, commentary on the bottlenecks and pitfalls frequently encountered when attempting to establish a co-culture with sugar-secreting cyanobacteria and a novel heterotrophic partner is included. This protocol provides a resource for researchers attempting to establish a new pair of co-cultured microbes that includes a cyanobacterium and a heterotrophic microbe.

This protocol provides a comprehensive guideline for the setup and quantitative monitoring of co-cultures, including photoautotrophic sugar-secreting cyanobacteria and heterotrophic yeasts.

With the increasing demand for sustainable biotechnologies, mixed consortia containing a phototrophic microbe and heterotrophic partner species are being ...

Genetic Modification of Cyanobacteria by Conjugation Using the CyanoGate Modular Cloning Toolkit

Cyanobacteria are a diverse group of prokaryotic photosynthetic organisms that can be genetically modified for the renewable production of useful industrial commodities. Recent advances in synthetic biology have led to development of several cloning toolkits such as CyanoGate, a standardized modular cloning system for building plasmid vectors for subsequent transformation or conjugal transfer into cyanobacteria. Here we outline a detailed method for assembling a self-replicating vector (e.g., carrying a fluorescent marker expression cassette) and conjugal transfer of the vector into the cyanobacterial strains Synechocystis sp. PCC 6803 or Synechococcus elongatus UTEX 2973. In addition, we outline how to characterize the performance of a genetic part (e.g., a promoter) using a plate reader or flow cytometry.

Here, we present a protocol describing how to i) assemble a self-replicating vector using the CyanoGate modular cloning toolkit, ii) introduce the vector into a cyanobacterial host by conjugation, and iii) characterize transgenic cyanobacteria strains using a plate reader or flow cytometry.

Generation of Marked and Markerless Mutants in Model Cyanobacterial Species

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