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1Department of Molecular, Cellular, and Developmental Biology, University of Michigan-Ann Arbor
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A detailed protocol is described for imaging the real time formation of DNA repair complexes in Bacillus subtilis cells.
Klocko, A. D., Crafton, K. M., Walsh, B. W., Lenhart, J. S., Simmons, L. A. Imaging Mismatch Repair and Cellular Responses to DNA Damage in Bacillus subtilis. J. Vis. Exp. (36), e1736, doi:10.3791/1736 (2010).
Growing cultures of cells for microscopy
1. One or two days prior to imaging, prepare the B. subtilis strain containing the translational fusion protein you wish to visualize. Trial rounds of steps 1A and 1B will be necessary to determine which growth condition provides the best images of your strain. In most cases, cells should be imaged during exponential growth phase.
2. On the morning of imaging, place 10 mL of S750 1-3 medium into a 125 mL sterile Erlenmeyer flask.
3. Resuspend the cells in up to 2 mL of S750 medium from the LB agar plate with single colonies or the light confluent cells to obtain an inoculum.
4. Add enough S750 medium-cell inoculum to the Erlenmeyer flask containing 10 mL S750 medium for a culture with an initial optical density measured at 600 nm (OD600) of ~0.08 to 0.1. At this step it is important to have at least a 10-fold air to medium ratio. Thus, we use 10 to 12.5 mL of inoculated medium in a 125 mL Erlenmeyer flask.
5. Grow each culture at 30°C until the culture reaches an OD600 ~0.5-0.8. Shaking water baths are preferred with revolutions per minute from 150-200. Cultures challenged with DNA damaging or mismatch inducing agents should be at an early exponential growth phase OD600 such that the cells will reach the target OD600 with the additional period of growth that is required for treatment (see Table 1)
For example, 2-aminopurine requires one hour of treatment, and therefore a culture treated with 2-aminopurine should be at OD600~0.3-0.4, such that the one hour of treatment/growth results in an OD600~0.5-0.8, whereby cells are ready for imaging.
1. Pipette 300 μl of cultured cells into a 1.5 mL microcentrifuge tube.
2. If imaging of cellular membranes is desired, add 1:1000 dilution of FM4-64 membrane stain (Invitrogen) to each sample from a 1 mg/mL stock solution A titration of FM4-64 may be required to achieve the desired fluorescence signal. A typical titration range is 1:100, 1:1000, and 1:10,000.
3. Allow the samples to sit at room temperature for up to ten minutes or while the slide is being prepared with 1% agarose in 1X Spizizens medium as described below (see step 1 of "slide preparation").
4. If concentration of cells is required, we prefer filtering cells using a 0.2 μm filter with a vacuum apparatus. Cells can then be gently washed from the surface of the filter with PBS, another appropriate buffer or medium prior to visualization.
1. Prepare 1% agarose with 1X Spizizens.
Add 5 mL of 10X Spizizens stock to 45 ml ddH2O along with 0.5 g agarose, and microwave to melt. Hot molten agarose is difficult to pipette, and solidified agarose will not enter the pipette tip The agarose should be equilibrated in a water bath to a temperature of ~55-65°C for it to enter the pipette tip without solidifying within the tip.
2. Draw 20 μl of Spizizens with 1% melted agarose into the pipette tip. Pipette a single drop (approximately 1-2 μL) of the agarose solution into each slide well (we use 15-well slides, see Table 2) to form shallow agarose pads. The agarose will solidify immediately. Slides must also be made immediately prior to application of the cells. If the agarose pads are left at room temperature alone, they can dehydrate within minutes and become unusable.
Agarose pads should be centered and fill each well but have minimum height to avoid disrupting the coverslip; if droplets are too high (bubble-like) or if the sample spreads beyond the well boundary, simply swipe the well clean with a Kimwipe and apply a new agarose pad.
3. Apply the entire 300 μl sample (see step 6) to the slide, distributed on the top of each agarose pad. This amount of cells should be sufficient to cover each well on the slide.
4. Allow the slide loaded with cells to sit at room temperature (23°C) for approximately ten minutes. This step will allow for the cells to settle on the agarose pad becoming immobile and ready for imaging. For specialized needs this temperature may need to be changed, for example, when performing experiments that require a temperature shift4.
5. Aspirate away the excess medium from each well without removing the pads themselves.
6. Apply the coverslip to the slide, pressing firmly and evenly, but gently, across the area of the slide. Check to make sure that the cover slip is against the slide and that the cover slip is not raised above the slide.
Visualization of cells
1. Many different fluorescence microscopes will work well. It is critical to have a 100X oil immersion lens. The Simmons Lab uses:
2. Bring the cells into focus using white light.
3. Capture the image using appropriate filters for each fluorophore
Representative images are shown (Figure 1). GFP foci should be well defined, while FM4-64 staining should be bright and clear 4-7. GFP or membrane images can be pseudo-colored with any color, to allow for the highest quality image to be presented without losing any data.
Figure 1: Representative GFP images of B. subtilis translational fusion proteins. GFP proteins are shown in green, while FM4-64 membrane staining is shown in red. The white scale bar indicates 3 μm. (A) MutS-GFP [relevant genotype: mutS-gfp (spc), amyE::PspacmutL (cat)] in the presence of 600 μg/mL 2-aminopurine and 1 mM IPTG. FM4-64 signal is not presented in order to more clearly show the MutS-GFP foci. (B) MutL-GFP [relevant genotype: mutL::mutL-GFP (spc)] in the presence of 2-aminopurine. (C) DnaX-GFP [relevant genotype: dnaX::dnaX-GFP (spc)]. (D) RecA-GFP [relevant genotype: recA::recA-GFP (spc)] in the presence of 20 ng/mL mitomycin C. (E) TagC-GFP [relevant genotype: tagC::tagC-GFP (spc)] in the presence of 1 μg/mL mitomycin C.
|Treatment /concentration or amount||Function||Time of incubation|
|Mitomycin C (MMC) [20 to 150 ng/mL]||DNA alkylating agent||1 hour|
|2-aminopurine (2-AP) [600 μg/mL]||mismatch inducing||1 hour|
|Hydroxyurea (HU)||depletes dNTPs||3 hours|
|ultraviolet light (UV) 20 J/m2||Thymine-thymine dimers and 6-4 photoproducts||varies depending on source, usually 20 seconds|
|grays (ionizing radiation) 5 to 100 Gy||Double strand breaks, single strand breaks and base damage sites||varies depending on source|
Table 1. Summary of DNA damaging and mismatch inducing treatments
|Name of the reagent||Company||Catalogue number||Comments (optional)|
|Multitest slide, 15-well||MP Biomedicals||6041505E|
|Microscope Cover Glass||Fisher||12-544-B|
|Mitomycin C||Sigma||M0503-2MG||mutagen, wear gloves|
Table 2. List of specific reagents and equipment:
Trial and error are required to find exposure conditions for the highest quality images for each strain; we find that 1 millisecond is appropriate for white light images, while exposures of 100 to 2000 ms are appropriate for GFP (FITC) and FM4-64 (TRITC) images. Exposure time will vary depending on the imaging equipment used. We recommend the use of one strain per 15-well microscope slide for the simplest imaging, as pad quality and diffusion of cells from the pad border could complicate strain differentiation if multiple strains are present on the same slide. Image quality directly depends on the quality of the agarose pad where the bacteria are resting. The highest quality images will be captured from agarose pads where bacteria are side by side. Pads that cause the bacterial cells to clump, preventing a monolayer, will produce poor images. Pads that are too thick will produce a high background fluorescent signal, while pads that are dehydrated will not allow for the cells to rest properly. These agarose pad defects will usually result in very poor image quality. If a well is producing poor images, simply move on to the next well. It is ideal to capture between 50 and 200 cells per image. Fluorescence microscopy has provided a wealth of information detailing the cellular cues that direct the assembly of DNA repair and replication proteins into foci in vivo. With practice, this technique can be successfully applied to a variety of protein complexes in many bacterial species.
The authors wish to thank Drs. Philina S. Lee and Alan D. Grossman for initially training L.A.S. in fluorescence microscopy. The authors also thank Drs. Melanie Berkmen and Hajime Kobayashi for help and tips for imaging. This work was supported by start-up funds from the College of Literature, Science & Arts and from the Department of Molecular, Cellular, and Developmental Biology at the University of Michigan.
|Specific solution recipes1:|
|10x S750 salts
0.5 M MOPS
100 mM Ammonium Sulfate
50 mM Potassium Phosphate Monobasic
Filter sterilize, and wrap S750 media in foil prior to storage
0.2 M MgCl2
70 mM CaCl2
5 mM MnCl2
0.1 mM ZnCl2
100 μg/mL Thiamine HCl
2 mM HCl
0.5 mM FeCl3*
dH2O to final volume
*FeCl3 should be added last, to prevent precipitation.
After filter sterilization, wrap 100x Metal solution in foil.
1x S750 salts
40 μg/mL Tryptophan
40 μg/mL Phenylalanine
distilled H2O to final volume
|10x Spizizens (grams/L)
151.4 mM Ammonium Sulfate (20g/L)
803.8 mM Potassium Phosphate Monobasic (140g/L)
440.9 mM Potassium Phosphate Dibasic (60g/L)
34.0 mM Sodium Citrate (10g/L)
16.6 mM MgSO4 (2g/L)
dH2O to final volume
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