Competent Cell Selection Guide

Find more protocols and selection guides in the Molecular Biology Guide.

Sigma-Aldrich offers a range of Escherichia coli bacterial cells made competent with the highest efficiencies by optimized procedure specific to each strain. Choose from 24 new competent cells for a wide variety of applications, including protein expression, routine or difficult cloning, and library generation. Many trial sizes available!

Application Product No. Product Description Transformation Efficiency Genotype Blue White Screening Capable
for protein expression and DNA plasmid production CMC0001 SIG10 Chemically Competent Cells ≥ 1 × 108 F- mcrA Δ(mrr-hsdRMS-mcrBC) endA1 recA1 Φ80dlacZΔM15 ΔlacX74 araD139 Δ(ara,leu)7697 galU galK rpsL nupG λ- tonA Y
for protein expression and DNA plasmid production CMC0002 SIG10 F' Chemically Competent Cells ≥ 5 × 108 [F´ pro A+B+ lacIqZΔM15::Tn10 (TetR)] /mcrA Δ(mrr-hsdRMS-mcrBC) endA1 recA1 Φ80dlacZΔM15 ΔlacX74 araD139 Δ(ara, leu)7697 galU galK rpsL nupGλ tonA Y
for protein expression and DNA plasmid production CMC0003 SIG10 HIGH Electrocompetent Cells ≥ 5 × 109 F- mcrA Δ(mrr-hsdRMS-mcrBC) endA1 recA1 Φ80dlacZΔM15 ΔlacX74 araD139 Δ(ara,leu)7697galU galK rpsL nupG λ- tonA (StrR) Y
for protein expression and DNA plasmid production CMC0004 SIG10 MAX Electrocompetent Cells ≥ 2 × 1010 F- mcrA Δ(mrr-hsdRMS-mcrBC) endA1 recA1 Φ80dlacZΔM15 ΔlacX74 araD139 Δ(ara,leu)7697galU galK rpsL nupG λ- tonA (StrR)  Y
for general cloning & library production CMC0005 SIG10 F' MAX Electrocompetent Cells ≥ 2 × 1010 [F´ pro A+B+ lacIqZΔM15::Tn10 (TetR)] /mcrA Δ(mrr-hsdRMS-mcrBC) endA1 recA1 Φ80dlacZΔM15 ΔlacX74 araD139 Δ(ara, leu)7697 galU galK rpsL nupG λ- tonA (StrR)  Y
for general cloning & library production CMC0006 SIG10 ULTRA Electrocompetent Cells
≥ 4 × 1010 F- mcrA Δ(mrr-hsdRMS-mcrBC) endA1 recA1 Φ80dlacZΔM15 ΔlacX74 araD139 Δ(ara,leu)7697galU galK rpsL nupG λ- tonA (StrR)  Y
for general cloning & library production CMC0007 SIG10 5a Chemically Competent Cells ≥ 1 × 108 fhuA2Δ(argF-lacZ)U169 phoA glnV44 Φ80 Δ(lacZ)M15 gyrA96 recA1 relA1 endA1 thi-1 hsdR17 Y
for plasmid production using unstable DNA CMC0008 STEADY Chemically Competent Cells > 1 × 107 recA13 supE44 ara-14 galK2 lacY1 proA2 rpsL20(StrR) xyl-5 λ– leu mtl-1 F– mcrB mrr hsdS20(rB–, mB–) N
for plasmid production using unstable DNA CMC0009 STEADY Electrocompetent Cells > 1 × 107 recA13 supE44 ara-14 galK2 lacY1 proA2 rpsL20(StrR) xyl-5 λ– leu mtl-1 F– mcrB mrr hsdS20(rB–, mB–) N
for making Uracil-containing DNA for mutagenesis CMC0010 CHANGER Electrocompetent Cells 1 × 109 [F’ Tra+ Pil+ (CamR)] ung-1 relA1 dut-1 thi-1 spoT1 mcrA N
for BAC & cosmid cloning CMC0011 XLDNA V2 Electrocompetent cells
≥ 1 × 1010 F- mcrA Δ(mrr-hsdRMS-mcrBC) endA1 recA1 Φ80dlacZΔM15 ΔlacX74 araD139 Δ(ara,leu)7697 galUgalK rpsL nupG (attL araC-PBAD-trfA250 bla attR) λ- Y
for BAC & cosmid cloning CMC0012 XLDNA SIG10 Electrocompetent cells ≥ 1 × 1010 F - pro A+B+ lacIqZΔM15::Tn10 (TetR)] /mcrA Δ(mrr-hsdRMS-mcrBC) endA1 recA1 Φ80dlacZΔM15 ΔlacX74 araD139 Δ(ara, leu)7697 galU galK rpsL nupG λ- tonA (StrR) N
for production of biotinylated proteins CMC0013 BIOTINYLATER F' Electrocompetent Cells >1 ×1010 MC1061 [F´ pro A+B+ lacIqZΔM15::Tn10 (TetR)] araD139 ∆(ara-leu)7696 ∆(lac)l74 galU galK hsdR2(rΚ- mΚ+) mcrB1 rpsL (StrR) birA N
for protein expression CMC0014 BL21(DE3) Chemically Competent Cells ≥ 1 × 107 F – ompT hsdSB (rB- mB-) gal dcm (DE3) N
for protein expression CMC0015 BL21(DE3) pLysE Chemically Competent Cells ≥ 1 × 107 F – ompT hsdSB (rB- mB-) gal dcm (DE3) N
for protein expression CMC0016 BL21(DE3) Electrocompetent Cells
≥ 5 x 109 F – ompT hsdSB (rB- mB-) gal dcm (DE3) N
for the highest protein expression CMC0017 OverExpress C41(DE3) Chemically Competent Cells ≥ 1 × 106 F – ompT hsdSB (rB- mB-) gal dcm (DE3)  N
for the highest protein expression CMC0018 OverExpress C41(DE3) pLysS Chemically Competent Cells ≥ 1 × 106 F – ompT hsdSB (rB- mB-) gal dcm (DE3) pLysS (CmR) N
for the highest protein expression CMC0019 OverExpress C43(DE3) Chemically Competent Cells ≥ 1 × 106 F – ompT hsdSB (rB- mB-) gal dcm (DE3) N
for the highest protein expression CMC0020 OverExpress C43(DE3) pLysS Chemically Competent Cells ≥ 1 × 106 F – ompT hsdSB (rB- mB-) gal dcm (DE3) pLysS (CmR) N
for the highest protein expression CMC0021 OverExpress C41(DE3) Electrocompetent Cells
≥ 1 × 109 F – ompT hsdSB (rB- mB-) gal dcm (DE3) N
for the highest protein expression CMC0022 OverExpress C43(DE3) Electrocompetent Cells ≥ 1 × 109 F – ompT hsdSB (rB- mB-) gal dcm (DE3) N
for controlled protein expression CMC0023 CONTROLLER SIG10 Chemically Competent Cells > 1 × 109 mcrA Δ(mrr-hsdRMS-mcrBC) endA1 recA1 ɸ80dlacZΔM15 ΔlacX74 araD139 Δ (ara,leu)7697 galU galK rpsL (StrR) nupG λ− tonA Mini-F lacIq1 (GentR) N
for controlled protein expression CMC0024 CONTROLLER BL21(DE3) Chemically Competent Cells > 1 × 107 F- ompT hsdSB (rB- mB-) gal dcm (DE3) Mini-F lacIq1(GentR) N

Guide to E. coli Markers

In the descriptions below, the markers are described as the genes or gene products.
dam Methylates A of GATC sites. The dam methyl pattern is used to distinguish old (methylated) from new (unmethylated) strands for damage-repair purposes (repair the unmethylated strand). Dam methylation is also used to time replication (don’t replicate until both strands are dam methylated). This methylation interferes with several restriction enzymes, including BclI and ClaI.
dcm Methylates CCWGG.
dnaJ Chaperonin. Mutants can stabilize some proteins in E. coli.
dut dUTPase, an enzyme that prevents the incorporation of uracil into DNA by destroying dUTP. Dut-ung double mutants accumulate a significant amount of uracil in their DNA.
e14 A prophage-like element, present in K-12 but missing from many derivatives. e14 carries the mcrA gene, so e14- strains are McrA-.
endA The gene for the end1 nuclease, the primary endonuclease of E coli.
F A self-transmissible plasmid (100 kb) that confers the ability to make pili (i.e. be "male") and thus to be infected by male-specific phage like M13.
F' An F plasmid that picked up some chromosomal DNA from E. coli. The F’ can have its own genotype. Two popular models code for the lacβ peptide (ΔM15), or LacIQ.
hsdRSM Restriction system that methylates host DNA in specific sequences and cleaves DNA that is not as methylated. The R gene is the endonuclease, the M gene is the methylase, and the S gene is required for each to function. Thus hsdR mutants don’t have the endonuclease function, but still methylate. HsdS mutants do not have the endonuclease or the methylase. Two popular models are the K system of E. coli K, and the B system of E. coli B.
lacIQ Overproduces the lacI gene product, a repressor of the lac operon.
lacY Lactose permease. LacY mutants cannot be used in blue/white screening, but are used to regulate gene expression in IPTG-induced gene expression.
lacZ β-galactosidase gene. The Δ(lacZ)M15 mutation expresses the β fragment. This is commonly found on F’ elements or in a defective 80 φ prophage. Many plasmids express the α fragment. The two together form a functional β-galactosidase.
Δlac There are three common deletions involving the entire lacZYA operon in addition to some flanking DNA: ΔU169, Δ X111, and ΔX74.
Ion A protease responsible for degrading aberrant proteins and for turning off sulA function. E. coli B naturally lacks lon.
mal B The malB region encompasses the genes malEFG and malK lamB malM. Δ (malB) deletes most or all of this entire region. LamB is the receptor for lambda phage.
mcr A Restricts DNA with methylcyctosine in certain sequences. McrA is lost when prophage e14 is lost.
mcr BC A system that restricts DNA containing methylcytosine in certain sequences. Δ(mcrC-mrr) deletes six genes: mcrC-mcrB-hsdS-hsdM-hsdR-mrr
mrr A methylcytosine and methyladenine restriction system.
recA Required for most homologous recombination pathways.
recB Required for ExoV function. Recombination deficient.
recC Required for ExoV function.
recD Required for ExoV function. RecD mutants stabilize inverted repeats, but interfere with plasmid maintenance.
recF Recombination gene required for interplasmid homologous recombination.
recJ Recombination gene required for interplasmid homologous recombination.
rpoH (or htpR heat shock transcription factor) Required to express some heat shock proteases.
sbcB Required for Exonuclease I function. Strains carrying recB recC and sbcB are usually also sbcC. These quadruple mutant strains are recombination-proficient and propagate inverted repeats in λ, but plasmid replication is aberrant.
sbcC Helps (with sbcB) to supress the effect of a recBC mutant. sbcC mutants are Rec+ and stably propagate inverted repeats in plasmids.
supE (or glnV) Mutant tRNA inserts glutamine at UAG codons, suppressing UAG mutations in the reading frame. SupE is required for the lytic growth of some phage mutants.
supF (or tyrT) Mutant tRNA inserts tyrosine at UAG codons, thus suppressing the effect of UAG mutations in reading frames. This mutation is required for the lytic growth of some λ phage, such as λgt11.
traD Conjugation defective mutant of the F plasmid.
ung Mutant in uracil N-glycosylase, an enzyme that removes uracil from DNA. An ung mutant allows uracil to persist in DNA.
φ80dlacΔM15 Lysogenic for a defective prophage derivative of φ80. The lacZ ΔM15 allele supplies the beta peptide for blue white screening.