Generating Chemically Competent Cells

Risk Assessment

The following protocol has been taken from [http://openwetware.org/index.php?title=TOP10_chemically_competent_cells&oldid=288222 Openwetware.org]

Overview

This protocol is a variant of the Hanahan protocol using CCMB80 buffer for DH10B, TOP10 and MachI strains. It builds on Example 2 of the Bloom05 patent as well. This protocol has been tested on TOP10, MachI and BL21(DE3) cells. See Bacterial Transformation for a more general discussion of other techniques. Jesse '464 patent describes using this buffer for DH5α cells. The Bloom04 patent describes the use of essentially the same protocol for the Invitrogen Mach 1 cells.

This is the chemical transformation protocol used by Tom Knight and the [http://partsregistry.org Registry of Standard Biological Parts].

Materials

• Detergent-free, sterile glassware and plasticware (see procedure)
• Table-top OD600nm spectrophotometer
• SOB

SOB

• 0.5% (w/v) yeast extract
• 2% (w/v) tryptone
• 10 mM NaCl
• 2.5 mM KCl
• 20 mM MgSO₄

Per liter:

• 5 g yeast extract
• 20 g tryptone
• 0.584 g NaCl
• 0.186 g KCl
• 2.4 g MgSO₄

• Some formulations of SOB use 10 mM MgCl₂ and 10 mM MgSO₄ instead of 20 mM MgSO₄.
• Adjust to pH 7.5 prior to use. This requires approximately 25 ml of 1M NaOH per liter.

CCMB80 buffer

• 10 mM KOAc pH 7.0 (10 ml of a 1M stock/L or 0.98g/L)
• 80 mM CaCl₂.2H₂O (11.8 g/L)
• 20 mM MnCl₂.4H₂O (4.0 g/L)
• 10 mM MgCl₂.6H₂O (2.0 g/L or 5ml of 2M stock/L)
• 10% glycerol (100 ml/L)
• Fill up with DI water AFTER regulating pH

• adjust pH DOWN to 6.4 with 0.1N HCl if necessary
  • adjusting pH up will precipitate manganese dioxide from Mn containing solutions.
• sterile filter and store at 4°C
• slight dark precipitate appears not to affect its function

Procedure

Preparing glassware and media

Eliminating detergent

Detergent is a major inhibitor of competent cell growth and transformation. Glass and plastic must be detergent free for these protocols. The easiest way to do this is to avoid washing glassware, and simply rinse it out. Autoclaving glassware filled 3/4 with DI water is an effective way to remove most detergent residue. Media and buffers should be prepared in detergent free glassware and cultures grown up in detergent free glassware.

Prechill plasticware and glassware

Prechill 250mL centrifuge tubes and screw cap tubes before use.

Preparing seed stocks

1. Streak TOP10 cells on an SOB plate and grow for single colonies at 23°C
   • room temperature works well
2. Pick single colonies into 2 ml of SOB medium and shake overnight at 23°C
   • room temperature works well
3. Add glycerol to 15%
4. Aliquot 1 ml samples to Nunc cryotubes
5. Place tubes into a zip lock bag, immerse bag into a dry ice/ethanol bath for 5 minutes
   • This step may not be necessary
6. Place in -80°C freezer indefinitely.

Preparing competent cells

1. Inoculate 250 ml of SOB medium with 1 ml vial of seed stock and grow at 20°C to an OD600nm of 0.3
   • This takes approximately 16 hours.
   • Controlling the temperature makes this a more reproducible process, but is not essential.
   • Room temperature will work. You can adjust this temperature somewhat to fit your schedule
   • Aim for lower, not higher OD if you can't hit this mark
2. Centrifuge at 3000g at 4°C for 10 minutes in a flat bottom centrifuge bottle.
   • Flat bottom centrifuge tubes make the fragile cells much easier to resuspend
   • It is often easier to resuspend pellets by mixing before adding large amounts of buffer
3. Gently resuspend in 80 ml of ice cold CCMB80 buffer
   • sometimes this is less than completely gentle. It still works.
4. Incubate on ice 20 minutes
5. Centrifuge again at 4°C and resuspend in 10 ml of ice cold CCMB80 buffer.
6. Test OD of a mixture of 200 μl SOC and 50 μl of the resuspended cells.
7. Add chilled CCMB80 to yield a final OD of 1.0-1.5 in this test.
8. Incubate on ice for 20 minutes
9. Aliquot to chilled screw top 2 ml vials or 50 μl into chilled microtiter plates
10. Store at -80°C indefinitely.
    • Flash freezing does not appear to be necessary
11. Test competence (see below)
12. Thawing and refreezing partially used cell aliquots dramatically reduces transformation efficiency by about 3x the first time, and about 6x total after several freeze/thaw cycles.

Measurement of competence

1. Transform 50 μl of cells with 1 μl of standard pUC19 plasmid (Invitrogen)
   • This is at 10 pg/μl or 10^-5 μg/μl
   • This can be made by diluting 1 μl of NEB pUC19 plasmid (1 μg/μl, NEB part number N3401S) into 100 ml of TE
2. Hold on ice 0.5 hours
3. Heat shock 60 sec at 42C
4. Add 250 μl SOC
5. Incubate at 37 C for 1 hour in 2 ml centrifuge tubes rotated
   • using 2ml centrifuge tubes for transformation and regrowth works well because the small volumes flow well when rotated, increasing aeration.
   • For our plasmids (pSB1AC3, pSB1AT3) which are chloramphenicol and tetracycline resistant, we find growing for 2 hours yields many more colonies
   • Ampicillin and kanamycin appear to do fine with 1 hour growth
6. Plate 20 μl on AMP plates using sterile 3.5 mm glass beads
   • Good cells should yield around 100 - 400 colonies
   • Transformation efficiency is (dilution factor=15) x colony count x 10⁵/µgDNA
   • We expect that the transformation efficiency should be between 5x10⁸ and 5x10⁹ cfu/µgDNA

References

<Biblio>

1. Hanahan91 pmid=1943786
2. Reusch86 pmid=3536850
3. Addison04 pmid=15470891
4. Bloom04 US Patent 6,709,852
5. Bloom05 US Patent 6,855,494
6. Jesse05 US Patent 6,960,464

</Biblio>