Work in our lab is supported by CAREER award MCB0551798 from the National Science Foundation and AREA grant R15GM86839 from the NIGMS-NIH.

1. Growth and UV Irradiation.
<ol>
	<li>200&mu;l of a fresh overnight culture containing the plasmid pBR322 grown in Davis medium1 supplemented with 0.4% glucose, 0.2% casamino acids, and 10 &mu;g/ml thymine (DGCthy medium) and 100 &mu;g/ml ampicillin is pelleted. The cell pellet is then resuspended in 200&mu;l DGCthy medium lacking ampicillin and used to inoculate 20 ml of DGCthy medium.</li>
	<li>Cultures are grown without ampicillin selection in a shaking incubator at 37&deg; C to an OD600 of 0.5 (~ 5 x 108 cells/ml). Growth without ampicillin avoids selection against abnormal or unproductive replication intermediates that may arise in some mutants. In addition, if using UV light to induce damage, the removal of the ampicillin from the media is necessary because it absorbs strongly at these wavelengths and shields the cells, reducing the effective dose of UV.</li>
	<li>Working under yellow lights, the culture is placed in a 15cm diameter Petri dish on a rotating platform for agitation. Our cultures are placed at a distance from a 15-watt germicidal lamp that produces an exposure rate of ~1 J/m2/sec, which is measured using a UVC photometer. Cultures are irradiated with 50 J/m2 and then placed immediately back into the shaking 37&deg; C incubator for the duration of the experiment. This dose produces, on average, 1 cyclobutane pyrimidine dimer every 4.5 kb of ssDNA . The yellow lighting prevents photoreactivation-reversal of cyclobutane pyrimidine dimers by photolyase.</li>
</ol>
2. DNA Isolation.
<ol>
	<li>At times when replication intermediates are to be examined, a 0.75 ml aliquot of the culture is placed into 0.75 ml ice cold NET30 Buffer (100 mM NaCl, 10 mM Tris, pH 8.0, 30 mM EDTA) and placed on ice until the end of the time course. We typically run a 90 minute time course, with samples examined at 0, 15, 30, 45, 60, and 90 minutes. The EDTA and cold temperature serve to effectively stop replication and nucleotide excision repair.</li>
	<li>Each sample is then pelleted, resuspended in 150 &mu;l of 1.5 mg/ml lysozyme and 0.2 mg/ml RNaseA in TE (10 mM Tris, pH 8.0, 1 mM EDTA), and lysed at 37&deg;C for 20 min. At this time, 10&mu;l of proteinase K (10mg/ml) and 10&mu;l of 20% sarkosyl are added and the incubation is allowed to continue for 1 hr. Proteinase K and sarkosyl help to release DNA fragments that may be associated with the membrane or proteins before phenol extraction occurs. Since actively replicating DNA is often bound to protein or membrane complexes, this helps increase the yield of replication fragments that are recovered.</li>
	<li>Samples are then extracted by adding 2 volumes of phenol to each sample and the tubes are gently inverted for 5 minutes. Then 2 volumes of chloroform/isoamyl alcohol (24/1) are added and the tubes are gently inverted again for 5 minutes.</li>
	<li>The samples are centrifuged at 14,000 rpm in a microcentrifuge for 5 minutes and the top aqueous phase of each sample is removed, and placed into a fresh tube. Then 4 volumes of chloroform/isoamyl alcohol (24/1) are added, the tubes are gently inverted for 5 minutes, and centrifuged again at 14,000 rpm for 5 minutes.</li>
	<li>The top, aqueous phase in each sample is then dialyzed for 1 hour by spotting 100&mu;l of each sample on a 47mm Whatman 0.025 &mu;m pore disk which floats on 250 ml of 0.1X TE in a beaker. Because replicating structures with single strand regions or branch points are more susceptible to shearing, we typically cut our pipette tips off with a razor blade to make the mouth wider and minimize shearing forces. In general, pipetting should be kept to a minimum until after the DNA has been digested with restriction enzymes.</li>
	<li>Each sample is then digested with PvuII (New England Biolabs) which linearizes the plasmid just downstream from the origin of replication.</li>
	<li>Prior to loading in the agarose gel, 100&mu;l chloroform and 20&mu;l of 6X loading dye that contains Bromophenol Blue and Xylene Cyanol are added to each sample and mixed. Then, 40&mu;l of the aqueous phase containing the loading dye is loaded into the gel. To minimize structural biases, structural artifacts, and DNA shearing, this isolation procedure does not include any steps to enrich for single strand regions, precipitate, or concentrate the DNA samples following cell lysis.</li>
</ol>
3. 2D Gel and Southern Analysis.
<ol>
	<li>The 2-dimensional agarose-gel analysis is modified from 3. For the 1ST dimension, the restricted DNA samples are run through a 0.4% agarose gel in 1X TBE at 1V/cm. One Liter of 10X TBE stock solution contains:
 <ul>
		<li>108 g Tris Base</li>
		<li>55 g Boric Acid</li>
		<li>40 ml 0.5M EDTA (pH 8.0)</li>
 </ul>
</li>
	<li>A lambda Hind III size marker is loaded in the first lane, then samples are loaded in every other lane. We typically run the first dimension ~12-15 hrs. Skipping lanes makes it easier to slice the lanes out to cast in the second dimension. The low voltage and low percent agarose gel serves to separate the DNA fragments primarily based upon size.</li>
	<li>For the second dimension, gel lanes are sliced out of the first dimension gel using a large butcher's knife. The first lane containing the lambda Hind III marker can be stained with ethidium bromide. Using the lambda Hind III marker as a guide, crop and discard the region of the gel that is below where the linearized plasmid is expected to run. Under these conditions, we find that linearized pBR322 runs slightly above the xylene cyanol stain.</li>
	<li>To cast the second dimension, each lane is placed horizontally across the top of an empty gel caster. A solution of 1.0% agarose in 1X TBE is prepared and cooled to 55&deg;C. At this time, the gel solution is poured in to cast the second dimension, making sure to completely cover the gel slices. Once the gel has set, it is run at 6.5V/cm in an electrophoresis unit that allows the buffer to recirculate. We typically run the second dimension ~5.5-7 hrs. The high voltage and high percent agarose gel effectively separates the DNA fragments based on their shape, as well as size. Nonlinear shapes run more slowly through the second dimension.</li>
	<li>Following electrophoresis, the gel is then rinsed in H2O, washed twice in 400ml of 0.25M hydrochloric acid for 15 minutes, rinsed with H2O;, and then washed twice in 400 ml 0.4M NaOH for 30 minutes. The acid washes serve to partially nick the DNA molecules into smaller fragments that transfer more efficiently. This step is necessary due to the large size and unusual shapes of the DNA replication intermediates.</li>
	<li>The DNA in the gels is then transferred to a Hybond N+ nylon membrane 4. We use a downward alkali transfer system with 0.4M NaOH. Briefly, two sheets of blotting paper soaked in 0.4M NaOH are placed on a large stack of paper towels. The nylon membrane is also soaked in 0.4M NaOH and placed on top of the blotting paper. The gel is then carefully layered on top of this, followed by another piece of blotting paper, wetted in the NaOH solution. Finally, a long piece of wetted blotting paper is layered across the top and its two ends are placed into a dishes containing 1 L of 0.4M NaOH solution to serve as a wick. We typically let the DNA transfer for 6-12 hrs.</li>
	<li>The nylon membrane is removed, washed 20-30 sec in 5X SSC Buffer, and placed in a hybridization roller bottle containing 10.5ml Prehybridization Solution. Then, the membrane is incubated at 42&deg;C with rotation for more than 6hrs. Prehybridization solution:
	<ul>
		<li>5.0 ml formamide</li>
		<li>0.5 ml 20% SDS</li>
		<li>2.5 ml 20X SSC*</li>
		<li>2.0 ml 50X Denhardt's*</li>
		<li>0.5 ml Salmon Sperm DNA (10mg/ml) *recipes for SSC and Denhardt's can be found in 5</li>
 </ul>
 </li>
	<li>During the prehybridization period, 1 &mu;g of pBR322 plasmid is labeled with 32P by nick translation according to the protocol supplied by Roche using alpha labeled [32-P]-dCTP. The radiolabeled probe (100&mu;l) is denatured by incubating at 98&deg; C for 10 minutes in a screw cap microcentrifuge tube, and then placed immediately on ice.</li>
	<li>The denatured probe is added to 5.9 ml hybridization solution, which contains:
 	<ul>
		<li>3.0 ml formamide</li>
		<li>1.5 ml SSC</li>
		<li>1.0 ml H2O</li>
		<li>0.15 ml 50X Denhardt's</li> 
		<li>0.10 ml 20% SDS</li>
		<li>0.15 ml 10mg/ml Salmon Sperm DNA</li>
	</ul>
</li>
	<li>The prehybridization solution is poured out of the roller bottle and the hybridization solution is poured in. The roller bottle is then returned to the 42&deg; C incubator and rotated for more than 12hrs.</li>
	<li>The hybridization solution is poured out of the roller bottle. Then, the blot is washed 4 times for 20 minutes in the roller bottle with ~150 ml of a wash solution containing 0.5X SSC, 0.1% SDS at 42&deg; C with rotation.</li>
	<li>Following the last wash, the membrane is placed on a paper towel until the liquid has visibly disappeared. The membrane is then wrapped in polyvinyl-chloride plastic wrap and exposed to a phosphorimager screen. We visualize and quantify the radioactivity using a Storm 840 and its associated ImageQuant Software.</li>
</ol>
4. Representative Results:
<img src="/files/ftp_upload/2220/2220fig1.jpg" alt="Figure 1" /> Figure 1. Plasmid replication intermediates observed in the presence and absence of UV-induced DNA damage. The migration pattern of PvuII digested pBR322 plasmid observed by 2D-agarose-gel analysis is diagrammed. Non-replicating linear plasmids run as a linear 4.4-kb fragment. Replicating plasmids form Y-shaped structures that migrate slower than the non-replicating fragments due to their larger size and nonlinear shape. This migration pattern forms an arc that extends out from the linear region towards the well. Following UV-irradiation, double-Y or X-shaped molecules are observed in the cone region that migrates more slowly than the arc of Y-shaped molecules. An example of the Southern analysis of the 2D gel probed with 32P-labeled pBR322 is shown for cells immediately after UV irradiation and 15 minutes after UV irradiation (reproduced with permission from the publisher).

<ol>
	<li>Davis, B. 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+Isolation+of+Biochemically+Deficient+Mutants+of+Bacteria+by+Means+of+Penicillin.">The Isolation of Biochemically Deficient Mutants of Bacteria by Means of Penicillin.</a> Proc Natl Acad Sci U S A. 35, 1-10 (1949).</li><li>Courcelle, J., Donaldson, J. R., Chow, K. H., Courcelle, C. T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=DNA+Damage-Induced+Replication+Fork+Regression+and+Processing+in+Escherichia+coli.">DNA Damage-Induced Replication Fork Regression and Processing in Escherichia coli.</a> Science. 299, 1064-1067 (2003).</li><li>Friedman, K. L., Brewer, B. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Analysis+of+replication+intermediates+by+two-dimensional+agarose+gel+electrophoresis.">Analysis of replication intermediates by two-dimensional agarose gel electrophoresis.</a> Methods Enzymol. 262, 613-627 (1995).</li><li>Spivak, G., Hanawalt, P. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Determination+of+damage+and+repair+in+specific+DNA+sequences.">Determination of damage and repair in specific DNA sequences.</a> METHODS: A Companion to Methods in Enzymology. 7, 147-161 (1995).</li><li>Sambrook, J., Russell, D. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Molecular Cloning - A laboratory manual. , (2001).</li><li>Donaldson, J. R., Courcelle, C. T., Courcelle, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=RuvABC+is+required+to+resolve+holliday+junctions+that+accumulate+following+replication+on+damaged+templates+in+Escherichia+coli.">RuvABC is required to resolve holliday junctions that accumulate following replication on damaged templates in Escherichia coli.</a> J Biol Chem. 281, 28811-28821 (2006).</li><li>Donaldson, J. R., Courcelle, C. T., Courcelle, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=RuvAB+and+RecG+Are+Not+Essential+for+the+Recovery+of+DNA+Synthesis+Following+UV-Induced+DNA+Damage+in+Escherichia+coli.">RuvAB and RecG Are Not Essential for the Recovery of DNA Synthesis Following UV-Induced DNA Damage in Escherichia coli.</a> Genetics. 166, 1631-1640 (2004).</li><li>Chow, K. H., Courcelle, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=RecO+Acts+with+RecF+and+RecR+to+Protect+and+Maintain+Replication+Forks+Blocked+by+UV-induced+DNA+Damage+in+Escherichia+coli.">RecO Acts with RecF and RecR to Protect and Maintain Replication Forks Blocked by UV-induced DNA Damage in Escherichia coli.</a> J Biol Chem. 279, 3492-3496 (2004).</li><li>Belle, J. J., Casey, A., Courcelle, C. T., Courcelle, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Inactivation+of+the+DnaB+helicase+leads+to+the+collapse+and+degradation+of+the+replication+fork:+a+comparison+to+UV-induced+arrest.">Inactivation of the DnaB helicase leads to the collapse and degradation of the replication fork: a comparison to UV-induced arrest.</a> J Bacteriol. 189, 5452-5462 (2007).</li><li>Chow, K. H., Courcelle, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=RecBCD+and+RecJ/RecQ+initiate+DNA+degradation+on+distinct+substrates+in+UV-irradiated+Escherichia+coli.">RecBCD and RecJ/RecQ initiate DNA degradation on distinct substrates in UV-irradiated Escherichia coli.</a> Radiat Res. 168, 499-506 (2007).</li><li>Al-Hadid, Q., Ona, K., Courcelle, C. T., Courcelle, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=RecA433+cells+are+defective+in+recF-mediated+processing+of+disrupted+replication+forks+but+retain+recBCD-mediated+functions.">RecA433 cells are defective in recF-mediated processing of disrupted replication forks but retain recBCD-mediated functions.</a> Mutat Res. 645, 19-26 (2008).</li></ol>

No conflicts of interest declared.

Typical results obtained from wild type cells in the presence and absence of UV-induced damage are shown in Figure 1. In the absence of damage, ~1% of the total plasmid DNA can be found in the Y arc when cells are rapidly growing in exponential phase. Following irradiation, a transient increase in Y shaped molecules is observed as blocked replication forks accumulate at damaged sites. The X-shaped replication intermediates also transiently accumulate and persist until a time that correlates with when the lesions are rep...

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<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">	
		
		<div>
		<table border="1">
		<thead>
		<tr>
		<th>Material Name</th>
		<th>Type</th>
		<th>Company</th>
		<th>Catalogue Number</th>
		<th>Comment</th>
		</tr>
		</thead>
		<tbody>
		
<tr>
<td>An example of the Southern analysis of the 2D gel probed</td>
<td>&nbsp;</td>
<td>GE</td>
<td>G15T8</td>
<td>Yellow Lighting</td>
</tr>
<tr>
<td>15 &mu;watt germicidal lamp</td>
<td>&nbsp;</td>
<td>Sylvania</td>
<td>F20T12/GO</td>
<td>UV Lamp</td>
</tr>
<tr>
<td>Blak-Ray UV Intensity Meter 254nm</td>
<td>&nbsp;</td>
<td>Daigger</td>
<td>EF28195T</td>
<td>UVC photometer</td>
</tr>
<tr>
<td>0.025 &mu;m pore disks</td>
<td>&nbsp;</td>
<td>Whatman</td>
<td>VSWP04700</td>
<td>Floating dialysis disks</td>
</tr>
<tr>
<td>PvuII</td>
<td>&nbsp;</td>
<td>Fermentas</td>
<td>ER0632</td>
<td>Restriction Endonuclease</td>
</tr>
<tr>
<td>Nick-translation kit</td>
<td>&nbsp;</td>
<td>Roche Diagnostics</td>
<td>976776</td>
<td>To make 32P-labeled probe</td>
</tr>
<tr>
<td>Blotting Paper</td>
<td>&nbsp;</td>
<td>Whatman</td>
<td>3030-704</td>
<td>For Southern transfer</td>
</tr>
<tr>
<td>Nylon membrane</td>
<td>&nbsp;</td>
<td>GE Healthcare</td>
<td>RPN203S</td>
<td>For Southern transfer</td>
</tr>		</tbody>
		</table>
		</div>

visualization of uv-induced replication intermediates in e. coli using two-dimensional agarose-gel analysis

Inaccurate replication in the presence of DNA damage is responsible for the majority of cellular rearrangements and mutagenesis observed in all cell types and is widely believed to be directly associated with the development of cancer in humans. DNA damage, such as that induced by UV irradiation, severely impairs the ability of replication to duplicate the genomic template accurately. A number of gene products have been identified that are required when replication encounters DNA lesions in the template. However, a remaining challenge has been to determine how these proteins process lesions during replication in vivo. Using Escherichia coli as a model system, we describe a procedure in which two-dimensional agarose-gel analysis can be used to identify the structural intermediates that arise on replicating plasmids in vivo following UV-induced DNA damage. This procedure has been used to demonstrate that replication forks blocked by UV-induced damage undergo a transient reversal that is stabilized by RecA and several gene products associated with the RecF pathway. The technique demonstrates that these replication intermediates are maintained until a time that correlates with the removal of the lesions by nucleotide excision repair and replication resumes.

Watch this Scientific Journal Video about Visualization of UV-induced Replication Intermediates in E. coli using Two-dimensional Agarose-gel Analysis at JoVE.com

Visualization of UV-induced Replication Intermediates in E. coli using Two-dimensional Agarose-gel Analysis

We present a procedure by which two-dimensional agarose-gel analysis can be used to identify the structure of replication intermediates that occur following UV irradiation.

Visualization of UV-induced Replication Intermediates in E. coli using Two-dimensional Agarose-gel Analysis

Inaccurate replication in the presence of DNA damage is responsible for the majority of cellular rearrangements and mutagenesis observed in all cell types ...

visualization-uv-induced-replication-intermediates-e-coli-using-two

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10.6K Views.  Portland State University. We present a procedure by which two-dimensional agarose-gel analysis can be used to identify the structure of replication intermediates that occur following UV irradiation.

Video: Visualization of UV-induced Replication Intermediates in E. coli using Two-dimensional Agarose-gel Analysis

Transformation of Plasmid DNA into E. coli Using the Heat Shock Method

Transformation of plasmid DNA into E. coli using the heat shock method is a basic technique of molecular biology. It consists of inserting a foreign plasmid or ligation product into bacteria. This video protocol describes the traditional method of transformation using commercially available chemically competent bacteria from Genlantis. After a short incubation in ice, a mixture of chemically competent bacteria and DNA is placed at 42&deg;C for 45 seconds (heat shock) and then placed back in ice. SOC media is added and the transformed cells are incubated at 37&deg;C for 30 min with agitation. To be assured of isolating colonies irrespective of transformation efficiency, two quantities of transformed bacteria are plated. This traditional protocol can be used successfully to transform most commercially available competent bacteria. The turbocells from Genlantis can also be used in a novel 3-minute transformation protocol, described in the instruction manual.

Transformation of plasmid DNA into E. coli using the heat shock method is a basic technique of molecular biology. It consists of inserting a foreign ...

Assessing Neural Stem Cell Motility Using an Agarose Gel-based Microfluidic Device

While microfluidic technology is reaching a new level of maturity for macromolecular assays, cell-based assays are still at an infant stage1. This is largely due to the difficulty with which one can create a cell-compatible and steady microenvironment using conventional microfabrication techniques and materials. We address this problem via the introduction of a novel microfabrication material, agarose gel, as the base material for the microfluidic device. Agarose gel is highly malleable, and permeable to gas and nutrients necessary for cell survival, and thus an ideal material for cell-based assays. We have shown previously that agarose gel based devices have been successful in studying bacterial and neutrophil cell migration2. In this report, three parallel microfluidic channels are patterned in an agarose gel membrane of about 1mm thickness. Constant flows with media/buffer are maintained in the two side channels using a peristaltic pump. Cells are maintained in the center channel for observation. Since the nutrients and chemicals in the side channels are constantly diffusing from the side to center channel, the chemical environment of the center channel is easily controlled via the flow along the side channels. Using this device, we demonstrate that the movement of neural stem cells can be monitored optically with ease under various chemical conditions, and the experimental results show that the over expression of epidermal growth factor receptors (EGFR) enhances the motility of neural stem cells. Motility of neural stem cells is an important biomarker for assessing cells aggressiveness, thus tumorigenic factor3. Deciphering the mechanism underlying NSC motility will yield insight into both disorders of neural development and into brain cancer stem cell invasion.

We demonstrate that the over expression of epidermal growth factor receptors (EGFR) enhances the motility of neural stem cells(NSCs) using a novel agarose gel based microfluidic device. This technology can be readily adaptable to other mammalian cell systems where cell sources are scarce, such as human neural stem cells, and the turn around time is critical.

While microfluidic technology is reaching a new level of maturity for macromolecular assays, cell-based assays are still at an infant stage1. This is ...

Screening for Amyloid Aggregation by Semi-Denaturing Detergent-Agarose Gel Electrophoresis

Amyloid aggregation is associated with numerous protein misfolding pathologies and underlies the infectious properties of prions, which are conformationally self-templating proteins that are thought to have beneficial roles in lower organisms.  Amyloids have been notoriously difficult to study due to their insolubility and structural heterogeneity.  However, resolution of amyloid polymers based on size and detergent insolubility has been made possible by Semi-Denaturing Detergent-Agarose Gel Electrophoresis (SDD-AGE).  This technique is finding widespread use for the detection and characterization of amyloid conformational variants.  Here, we demonstrate an adaptation of this technique that facilitates its use in large-scale applications, such as screens for novel prions and other amyloidogenic proteins.  The new SDD-AGE method uses capillary transfer for greater reliability and ease of use, and allows any sized gel to be accomodated.  Thus, a large number of samples, prepared from cells or purified proteins, can be processed simultaneously for the presence of SDS-insoluble conformers of tagged proteins.

SDD-AGE is a useful technique for the detection and characterization of amyloid-like polymers in cells. Here we demonstrate an adaptation that makes this technique amenable to large-scale applications.

Amyloid aggregation is associated with numerous protein misfolding pathologies and underlies the infectious properties of prions, which are ...

Optical Scatter Microscopy Based on Two-Dimensional Gabor Filters

We demonstrate a microscopic instrument that can measure subcellular texture arising from organelle morphology and organization within unstained living cells. The proposed instrument extends the sensitivity of label-free optical microscopy to nanoscale changes in organelle size and shape and can be used to accelerate the study of the structure-function relationship pertaining to organelle dynamics underlying fundamental biological processes, such as programmed cell death or cellular differentiation. The microscope can be easily implemented on existing microscopy platforms, and can therefore be disseminated to individual laboratories, where scientists can implement and use the proposed methods with unrestricted access.
The proposed technique is able to characterize subcellular structure by observing the cell through two-dimensional optical Gabor filters. These filters can be tuned to sense with nanoscale (10's of nm) sensitivity, specific morphological attributes pertaining to the size and orientation of non-spherical subcellular organelles. While based on contrast generated by elastic scattering, the technique does not rely on a detailed inverse scattering model or on Mie theory to extract morphometric measurements. This technique is therefore applicable to non-spherical organelles for which a precise theoretical scatter description is not easily given, and provides distinctive morphometric parameters that can be obtained within unstained living cells to assess their function. The technique is advantageous compared with digital image processing in that it operates directly on the object's field transform rather than the discretized object's intensity. It does not rely on high image sampling rates and can therefore be used to rapidly screen morphological activity within hundreds of cells at a time, thus greatly facilitating the study of organelle structure beyond individual organelle segmentation and reconstruction by fluorescence confocal microscopy of highly magnified digital images of limited fields of view.
In this demonstration we show data from a marine diatom to illustrate the methodology. We also show preliminary data collected from living cells to give an idea of how the method may be applied in a relevant biological context.

We demonstrate a dark-field microscopy method based on Gabor-like filtering to measure subcellular dynamics within single living cells. The technique is sensitive to alterations in the structure of organelles, such as mitochondrial fragmentation.

We demonstrate a microscopic instrument that can measure subcellular texture arising from organelle morphology and organization within unstained living ...

Mutagenesis and Functional Selection Protocols for Directed Evolution of Proteins in E. coli

The efficient generation of genetic diversity represents an invaluable molecular tool that can be used to label DNA synthesis, to create unique molecular signatures, or to evolve proteins in the laboratory. Here, we present a protocol that allows the generation of large (>1011) mutant libraries for a given target sequence. This method is based on replication of a ColE1 plasmid encoding the desired sequence by a low-fidelity variant of DNA polymerase I (LF-Pol I). The target plasmid is transformed into a mutator strain of E. coli and plated on solid media, yielding between 0.2 and 1 mutations/kb, depending on the location of the target gene. Higher mutation frequencies are achieved by iterating this process of mutagenesis. Compared to alternative methods of mutagenesis, our protocol stands out for its simplicity, as no cloning or PCR are involved. Thus, our method is ideal for mutational labeling of plasmids or other Pol I templates or to explore large sections of sequence space for the evolution of activities not present in the original target. The tight spatial control that PCR or randomized oligonucleotide-based methods offer can also be achieved through subsequent cloning of specific sections of the library. Here we provide protocols showing how to create a random mutant library and how to establish drug-based selections in E. coli to identify mutants exhibiting new biochemical activities.

Mutagenesis and Functional Selection Protocols for Directed Evolution of Proteins in E. coli

Here we demonstrate a simple protocol to create a random mutant library for a given target sequence. We show how this method, which is performed in vivo in Escherichia coli, can be coupled with functional selections to evolve new enzymatic activities.

The efficient generation of genetic diversity represents an invaluable molecular tool that can be used to label DNA synthesis, to create unique molecular ...

Visualization of DNA Replication in the Vertebrate Model System DT40 using the DNA Fiber Technique

Maintenance of replication fork stability is of utmost importance for dividing cells to preserve viability and prevent disease. The processes involved not only ensure faithful genome duplication in the face of endogenous and exogenous DNA damage but also prevent genomic instability, a recognized causative factor in tumor development.
Here, we describe a simple and cost-effective fluorescence microscopy-based method to visualize DNA replication in the avian B-cell line DT40. This cell line provides a powerful tool to investigate protein function in vivo by reverse genetics in vertebrate cells1. DNA fiber fluorography in DT40 cells lacking a specific gene allows one to elucidate the function of this gene product in DNA replication and genome stability. Traditional methods to analyze replication fork dynamics in vertebrate cells rely on measuring the overall rate of DNA synthesis in a population of pulse-labeled cells. This is a quantitative approach and does not allow for qualitative analysis of parameters that influence DNA synthesis. In contrast, the rate of movement of active forks can be followed directly when using the DNA fiber technique2-4. In this approach, nascent DNA is labeled in vivo by incorporation of halogenated nucleotides (Fig 1A). Subsequently, individual fibers are stretched onto a microscope slide, and the labeled DNA replication tracts are stained with specific antibodies and visualized by fluorescence microscopy (Fig 1B). Initiation of replication as well as fork directionality is determined by the consecutive use of two differently modified analogues. Furthermore, the dual-labeling approach allows for quantitative analysis of parameters that influence DNA synthesis during the S-phase, i.e. replication structures such as ongoing and stalled forks, replication origin density as well as fork terminations. Finally, the experimental procedure can be accomplished within a day, and requires only general laboratory equipment and a fluorescence microscope.

DT40, a model vertebrate genetic system, provides a powerful tool to analyze protein function. Here we describe a simple method that allows qualitative analysis of parameters that influence DNA synthesis during the S-phase in DT40 cells at the single molecule level.

Maintenance of replication fork stability is of utmost importance for dividing cells to preserve viability and prevent disease. The processes involved not ...

Agarose Gel Electrophoresis for the Separation of DNA Fragments

Agarose gel electrophoresis is the most effective way of separating DNA fragments of varying sizes ranging from 100 bp to 25 kb1. Agarose is isolated from the seaweed genera Gelidium and Gracilaria, and consists of repeated agarobiose (L- and D-galactose) subunits2. During gelation, agarose polymers associate non-covalently and form a network of bundles whose pore sizes determine a gel&#39;s molecular sieving properties. The use of agarose gel electrophoresis revolutionized the separation of DNA. Prior to the adoption of agarose gels, DNA was primarily separated using sucrose density gradient centrifugation, which only provided an approximation of size. To separate DNA using agarose gel electrophoresis, the DNA is loaded into pre-cast wells in the gel and a current applied. The phosphate backbone of the DNA (and RNA) molecule is negatively charged, therefore when placed in an electric field, DNA fragments will migrate to the positively charged anode. Because DNA has a uniform mass/charge ratio, DNA molecules are separated by size within an agarose gel in a pattern such that the distance traveled is inversely proportional to the log of its molecular weight3. The leading model for DNA movement through an agarose gel is &#34;biased reptation&#34;, whereby the leading edge moves forward and pulls the rest of the molecule along4. The rate of migration of a DNA molecule through a gel is determined by the following: 1) size of DNA molecule; 2) agarose concentration; 3) DNA conformation5; 4) voltage applied, 5) presence of ethidium bromide, 6) type of agarose and 7) electrophoresis buffer. After separation, the DNA molecules can be visualized under uv light after staining with an appropriate dye. By following this protocol, students should be able to: 
 
1. Understand the mechanism by which DNA fragments are separated within a gel matrix 
2. Understand how conformation of the DNA molecule will determine its mobility through a gel matrix 
3. Identify an agarose solution of appropriate concentration for their needs 
4. Prepare an agarose gel for electrophoresis of DNA samples 
5. Set up the gel electrophoresis apparatus and power supply 
6. Select an appropriate voltage for the separation of DNA fragments 
7. Understand the mechanism by which ethidium bromide allows for the visualization of DNA bands 
8. Determine the sizes of separated DNA fragments 
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A basic protocol for the separation of DNA fragments using agarose gel electrophoresis is described.

Agarose gel electrophoresis is the most effective way of separating DNA fragments of varying sizes ranging from 100 bp to 25 kb1. Agarose is isolated from ...

Rapid Genetic Analysis of Epithelial-Mesenchymal Signaling During Hair Regeneration

Hair follicle morphogenesis, a complex process requiring interaction between epithelia-derived keratinocytes and the underlying mesenchyme, is an attractive model system to study organ development and tissue-specific signaling. Although hair follicle development is genetically tractable, fast and reproducible analysis of factors essential for this process remains a challenge. Here we describe a procedure to generate targeted overexpression or shRNA-mediated knockdown of factors using lentivirus in a tissue-specific manner. Using a modified version of a hair regeneration model 5, 6, 11, we can achieve robust gain- or loss-of-function analysis in primary mouse keratinocytes or dermal cells to facilitate study of epithelial-mesenchymal signaling pathways that lead to hair follicle morphogenesis. We describe how to isolate fresh primary mouse keratinocytes and dermal cells, which contain dermal papilla cells and their precursors, deliver lentivirus containing either shRNA or cDNA to one of the cell populations, and combine the cells to generate fully formed hair follicles on the backs of nude mice. This approach allows analysis of tissue-specific factors required to generate hair follicles within three weeks and provides a fast and convenient companion to existing genetic models.

Tissue-specific analysis of a hair follicle regeneration model using lentivirus to mediate gain- or loss-of-function.

Hair follicle morphogenesis, a complex process requiring interaction between epithelia-derived keratinocytes and the underlying mesenchyme, is an ...

Visualization and Analysis of mRNA Molecules Using Fluorescence In Situ Hybridization in Saccharomyces cerevisiae

The Fluorescence in situ Hybridization (FISH) method allows one to detect nucleic acids in the native cellular environment. Here we provide a protocol for using FISH to quantify the number of mRNAs in single yeast cells. Cells can be grown in any condition of interest and then fixed and made permeable. Subsequently, multiple single-stranded deoxyoligonucleotides conjugated to fluorescent dyes are used to label and visualize mRNAs. Diffraction-limited fluorescence from single mRNA molecules is quantified using a spot-detection algorithm to identify and count the number of mRNAs per cell. While the more standard quantification methods of northern blots, RT-PCR and gene expression microarrays provide information on average mRNAs in the bulk population, FISH facilitates both the counting and localization of these mRNAs in single cells at single-molecule resolution.

Visualization and Analysis of mRNA Molecules Using Fluorescence In Situ Hybridization in Saccharomyces cerevisiae

This protocol describes an experimental procedure for performing Fluorescence in situ Hybridization (FISH) for counting mRNAs in single cells at single-molecule resolution.

The Fluorescence in situ Hybridization (FISH) method allows one to detect nucleic acids in the native cellular environment. Here we provide a protocol for ...

Phage-mediated Delivery of Targeted sRNA Constructs to Knock Down Gene Expression in E. coli

RNA-mediated knockdowns are widely used to control gene expression. This versatile family of techniques makes use of short RNA (sRNA) that can be synthesized with any sequence and designed to complement any gene targeted for silencing. Because sRNA constructs can be introduced to many cell types directly or using a variety of vectors, gene expression can be repressed in living cells without laborious genetic modification. The most common RNA knockdown technology, RNA interference (RNAi), makes use of the endogenous RNA-induced silencing complex (RISC) to mediate sequence recognition and cleavage of the target mRNA. Applications of this technique are therefore limited to RISC-expressing organisms, primarily eukaryotes. Recently, a new generation of RNA biotechnologists have developed alternative mechanisms for controlling gene expression through RNA, and so made possible RNA-mediated gene knockdowns in bacteria. Here we describe a method for silencing gene expression in E. coli that functionally resembles RNAi. In this system a synthetic phagemid is designed to express sRNA, which may designed to target any sequence. The expression construct is delivered to a population of E. coli cells with non-lytic M13 phage, after which it is able to stably replicate as a plasmid. Antisense recognition and silencing of the target mRNA is mediated by the Hfq protein, endogenous to E. coli. This protocol includes methods for designing the antisense sRNA, constructing the phagemid vector, packaging the phagemid into M13 bacteriophage, preparing a live cell population for infection, and performing the infection itself. The fluorescent protein mKate2 and the antibiotic resistance gene chloramphenicol acetyltransferase (CAT) are targeted to generate representative data and to quantify knockdown effectiveness.

Phage-mediated Delivery of Targeted sRNA Constructs to Knock Down Gene Expression in E. coli

We describe a method to knock down gene expression in a growing population of E. coli cells using sequence-targeted sRNA expression cassettes delivered by an M13 phagemid vector.

Visualization of UV-induced Replication Intermediates in E. coli using Two-dimensional Agarose-gel Analysis

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Agarose Gel Electrophoresis for the Separation of DNA Fragments

Rapid Genetic Analysis of Epithelial-Mesenchymal Signaling During Hair Regeneration

Visualization and Analysis of mRNA Molecules Using Fluorescence In Situ Hybridization in Saccharomyces cerevisiae

Phage-mediated Delivery of Targeted sRNA Constructs to Knock Down Gene Expression in E. coli