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Method Article
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The goal of this protocol is to provide a detailed, step-by-step guide for assembling multi-gene constructs using the modular cloning system based on Golden Gate cloning. It also gives recommendations on critical steps to ensure optimal assembly based on our experiences.
The Golden Gate cloning method enables the rapid assembly of multiple genes in any user-defined arrangement. It utilizes type IIS restriction enzymes that cut outside of their recognition sites and create a short overhang. This modular cloning (MoClo) system uses a hierarchical workflow in which different DNA parts, such as promoters, coding sequences (CDS), and terminators, are first cloned into an entry vector. Multiple entry vectors then assemble into transcription units. Several transcription units then connect into a multi-gene plasmid. The Golden Gate cloning strategy is of tremendous advantage because it allows scar-less, directional, and modular assembly in a one-pot reaction. The hierarchical workflow typically enables the facile cloning of a large variety of multi-gene constructs with no need for sequencing beyond entry vectors. The use of fluorescent protein dropouts enables easy visual screening. This work provides a detailed, step-by-step protocol for assembling multi-gene plasmids using the yeast modular cloning (MoClo) kit. We show optimal and suboptimal results of multi-gene plasmid assembly and provide a guide for screening for colonies. This cloning strategy is highly applicable for yeast metabolic engineering and other situations in which multi-gene plasmid cloning is required.
Synthetic biology aims to engineer biological systems with new functionalities useful for pharmaceutical, agricultural, and chemical industries. Assembling large numbers of DNA fragments in a high-throughput manner is a foundational technology in synthetic biology. Such a complicated process can break down into multiple levels with decreasing complexity, a concept borrowed from basic engineering sciences1,2. In synthetic biology, DNA fragments usually assemble hierarchically based on functionality: (i) Part level: "parts" refers to DNA fragments with a specific function, such as a promoter, a coding sequence, a terminator, an origin of replication; (ii) Transcription units (TU) level: a TU consists of a promoter, a coding sequence, and a terminator capable of transcribing a single gene; (iii) Multi-gene level: a multi-gene plasmid contains multiple TUs frequently comprised of an entire metabolic pathway. This hierarchical assembly pioneered by the BioBrick community is the foundational concept for the assembly of large sets of DNAs in synthetic biology3.
In the past decade4,5,6,7, the Golden Gate cloning technique has significantly facilitated hierarchical DNA assembly2. Many other multi-part cloning methods, such as Gibson cloning8, ligation-independent cloning (SLIC)9, uracil excision-based cloning (USER)10, the ligase cycling reaction (LCR)11, and in vivo recombination (DNA Assembler)12,13, have also been developed so far. But Golden Gate cloning is an ideal DNA assembly method because it is independent of gene-specific sequences, allowing scar-less, directional, and modular assembly in a one-pot reaction. Golden Gate cloning takes advantage of type IIS restriction enzymes that recognize a non-palindromic sequence to create staggered overhangs outside of the recognition site2. A ligase then joins the annealed DNA fragments to obtain a multi-part assembly. Applying this cloning strategy to the modular cloning (MoClo) system has enabled the assemble of up to 10 DNA fragments with over 90% transformants screened containing the correctly assembled construct4.
The MoClo system offers tremendous advantages that have accelerated the design-build-test cycle of synthetic biology. Firstly, the interchangeable parts enable combinatorial cloning to test a large space of parameters rapidly. For example, optimizing a metabolic pathway usually requires cycling through many promoters for each gene to balance the pathway flux. The MoClo system can easily handle such demanding cloning tasks. Secondly, one needs to sequence the part plasmid but typically not the TU or the multi-gene plasmids. In most cases, screening by colony PCR or restriction digestion is sufficient for verification at the TU and multi-gene plasmid level. This is because cloning the part plasmid is the only step requiring PCR, which frequently introduces mutations. Thirdly, the MoClo system is ideal for building multi-gene complex metabolic pathways. Lastly, because of the universal overhangs, the part plasmids can be reused and shared with the entire bioengineering community. Currently, MoClo kits are available for plants14,15,5,16,17, fungi6,18,19,20,21,22, bacteria7,23,24,25,26,27, and animals28,29. A multi-kingdom MoClo platform has also been introduced recently30.
For Saccharomyces cerevisiae, Lee et al.6 have developed a versatile MoClo toolkit, an excellent resource for the yeast synthetic biology community. This kit comes in a convenient 96-well format and defines eight types of interchangeable DNA parts with a diverse collection of well-characterized promoters, fluorescent proteins, terminators, peptide tags, selection markers, origin of replication, and genome editing tools. This toolkit allows the assembly of up to five transcription units into a multi-gene plasmid. These features are valuable for yeast metabolic engineering, in which partial or entire pathways are over-expressed to produce targeted chemicals. Using this kit, researchers have optimized the production of geraniol, linalool31, penicillin32, muconic acid33, indigo34, and betalain35 in yeast.
Here we provide a detailed, step-by-step protocol to guide the use of the MoClo toolkit to generate multi-gene pathways for either episomal or genomic expression. Through extensive use of this kit, we have found that the accurate measurement of DNA concentrations is key to ensuring the equimolar distribution of each part in the Golden Gate reaction. We also recommend the T4 DNA ligase over the T7 DNA ligase because the former works better with larger numbers of overhangs36. Lastly, any internal recognition sites of BsmBI and BsaI must be removed or domesticated prior to assembly. Alternatively, one may consider synthesizing parts to remove multiple internal sites and to achieve simultaneous codon optimization. We demonstrate how to use this toolkit by expressing a five-gene pathway for β-carotene and lycopene production in S. cerevisiae. We further show how to knock out the ADE2 locus using the genome-editing tools from this kit. These color-based experiments were selected for easy visualization. We also demonstrate how to generate fusion proteins and to create amino acid mutations using Golden Gate cloning.
NOTE: The hierarchical cloning protocol offered in this toolkit can be divided into three major steps: 1. Cloning part plasmids; 2. Cloning transcription units (TUs); 3. Cloning multi-gene plasmids (Figure 1). This protocol starts from the primer design and ends with applications of the cloned multi-gene plasmid.
1. Primer design for cloning the part plasmid (pYTK001):
2. Cloning parts into the entry vector (pYTK001) to create part plasmids (Figure 2B)
3. Assembling part plasmids into "cassette" plasmids
NOTE: A cassette plasmid contains a user-defined transcription unit (TU) consisting of a promoter, a CDS, and a terminator. A cassette plasmid allows the expression of a single gene. If the cassette plasmids will be assembled into a multi-gene plasmid, then the first step is to determine the number and the order of TUs in the multi-gene plasmid. These will determine which connectors to use in the cassette plasmids since connectors link TUs in the multi-gene plasmid. The first TU's left connector should be ConLS, and the right connector of the last TU should be ConRE. They will overlap with ConLS' and ConRE' of multi-gene plasmids. The rest of the connectors should be in the increasing numerical order. For example, if the multi-gene plasmid contains four TUs, the connector combinations would be ConLS-TU1-ConR1, ConL1-TU2-ConR2, ConL2-TU3-ConR3, and ConL3-TU4-ConRE (Figure 1).
4. Assembling cassette plasmids into "multi-gene" plasmids:
NOTE: Multi-gene plasmids allow the expression of more than one gene. Depending on the downstream application, multi-gene plasmids could be replicative or integrative. Replicative plasmids have the yeast origin of replication; therefore, it can be stably maintained when yeast cell divides. Integrative plasmids do not have the yeast origin of replication. Instead, they have 5' and 3' homology arms allowing the integration of multiple genes into specific loci of the genome through homologous recombination.
5. Applying multi-gene plasmids for chromosomal or plasmid-based gene expression
Here the results of four replicative multi-gene plasmids for β-carotene (yellow) and lycopene (red) production. One integrative multi-gene plasmid for disrupting the ADE2 locus was constructed, the colonies of which are red.
Cloning CDSs into the entry vector (pYTK001)
ERG20 was amplified from the yeast genome and the three carotenoid genes crtE, crtYB, crtI
The MoClo based cloning kit developed by Lee et al. provides an excellent resource for quick assembly of one to five transcription units into a multi-gene plasmid either for replication or integration into the yeast genome. The use of this kit eliminates the time-consuming cloning bottleneck that frequently exists for expressing multiple genes in yeast.
We tested five different conditions for the digestion/ligation cycles of Golden Gate cloning with T4 DNA ligase. We found that 30 cycles of di...
The authors have nothing to disclose.
This work was funded by the Research Foundation for the State University of New York (Award #: 71272) and the IMPACT Award of University at Buffalo (Award #: 000077).
Name | Company | Catalog Number | Comments |
0.5 mm Glass beads | RPI research products | 9831 | For lysing yeast cells |
Bacto Agar | BD & Company | 214010 | Component of the yeast complete synthetic medium (CSM) |
Bacto Peptone | BD& Company | 211677 | Component of the yeast extract peptone dextrose medium (YPD) |
BsaI-HFv2 | New England Biolabs | R3733S | a highly efficient version of BsaI restriction enzyme |
Carbenicillin | Fisher Bioreagents | 4800-94-6 | Antibiotic for screening at the transcription unit level |
Chloramphenicol | Fisher Bioreagents | 56-75-7 | Antibiotic for screening at the entry vector level |
CSM-His | Sunrise Sciences | 1006-010 | Amino acid supplement of the yeast complete synthetic medium (CSM) |
Dextrose | Fisher Chemical | D16-500 | Carbon source of the yeast complete synthetic medium (CSM) |
Difco Yeast Nitrogen Base w/o Amino Acids | BD & Company | 291940 | Nitrogen source of the yeast complete synthetic medium (CSM) |
dNTP mix | Promega | U1515 | dNTPs for PCR |
Esp3I | New England Biolabs | R0734S | a highly efficient isoschizomer of BsmBI |
Frozen-EZ Yeast Trasformation II Kit | Zymo Research | T2001 | For yeast transformation |
Hexanes | Fisher Chemical | H302-1 | For carotenoid extraction from yeast cells |
Kanamycin | Fisher Bioreagents | 25389-94-0 | Antibiotic for screening at the multigene plasmid level |
LB Agar, Miller | Fisher Bioreagents | BP1425-2 | Lysogenic agar medium for E. coli culturing |
LB Broth, Miller | Fisher Bioreagents | BP1426-2 | Lysogenic liquid medium for E. coli culturing |
Lycopene | Cayman chemicals | NC1142173 | For lycopene quantification |
MoClo YTK | Addgene | 1000000061 | Depositing Lab: John Deuber |
Monarch Plasmid Miniprep Kit | New England Biolabs | T1010L | For plasmid purification from E.coli |
Nanodrop Spectrophotometer | Thermo Scientific | ND2000c | For measuring accurate DNA concentrations |
NotI-HF | New England Biolabs | R3189S | Restriction enzyme for integrative multigene plasmid linearization |
Nourseothricin Sulphate | Goldbio | N-500-100 | Antibiotic Selection marker for the pCAS plasmid used in this study |
Phusion HF reaction Buffer (5X) | New England Biolabs | B0518S | Buffer for PCR using Phusion polymerase |
Phusion High Fidelity DNA Polymerase | New England Biolabs | M0530S | High fidelity polymerase for all the PCR reactions |
pLM494 | Addgene | 100539 | Plasmid used to amplify crtI, crtYB and crtE used in this study |
Quartz Cuvette | Thermo Electron | 10050801 | For quantifing carotenoids |
T4 ligase | New England Biolabs | M0202S | Ligase for Golden Gate cloning |
Thermocycler | BIO-RAD | 1851148 | For performing all the PCR and cloning reactions |
Tissue Homogenizer | Bullet Blender | Model: BBX24 | For homogenization of yeast cells |
UV-Vis Spectrophotometer | Thermo Scientific | Genesys 150 | For quantifing carotenoids |
Yeast Extract | Fisher Bioreagents | BP1422-500 | Component of the yeast extract peptone dextrose medium (YPD) |
β-carotene | Alfa Aesar | AAH6010603 | For β-carotene quantification |
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