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E. coli B strains and its derivatives are commonly used for recombinant protein expression. The most prevalent strains for expressing cloned proteins in bacteria are BL21(DE3) and its variants. BL21(DE3) cells produce high levels of protein from the T7 promoter and can be used with high or low copy number plasmids, making BL21(DE3) competent cells the preferred strains for protein expression in bacteria.

Why use BL21 cells for protein expression?

BL21 E. coli yield high levels of protein due to their deficiency in two key proteases (lon and ompT). This permits high-level intracellular accumulation of heterologous proteins while minimizing protein degradation during purification. In addition, BL21 variants that exhibit control over protein expression are available, providing the opportunity to select a competent cell strain well matched to the chosen vector and/or target gene.

The BL21(DE3) strain was first published by Studier and Moffatt in 1986  for the pET expression system using the T7 promoter; pET expression kits with different vectors and competent cells are available.

Depending on the toxicity of the protein you are trying to express, you may choose a strain derived from BL21 E. coli that supports control of T7 RNA polymerase under either the lacUV5 or araBAD promoter, or one that expresses T7 lysozyme.

Learn more about bacterial strains for protein expression

BL21 competent cells selection guide

All Invitrogen BL21 cells for protein expression:

  • Express T7 RNA polymerase
  • Have transformation efficiencies of at least 1 x 107 cfu/µg
  • Are available in the One Shot format
  • Are supplied with SOC outgrowth medium and control plasmid
ProductBL21 cells One Shot kit box, 21 brown-capped tubes, 2 vials of growth media, and 1 tube of control DNABL21 cells One Shot kit box, 21 green-capped tubes, 2 vials of growth media, and 1 tube of control DNABL21 star E. coli One Shot kit box, 21 red-capped tubes, 1 vial of growth media and 1 tube of control DNABL21 pLysS E. coli One Shot kit box, 21 blue-capped tubes, 2 vials of growth media, and 1 tube of control DNABL21 AI E. coli One Shot kit box, 21 orange-capped tubes and 2 black-capped tubes

BL21 (DE3) Chemically Competent E. coliBL21(DE3)pLysS Chemically Competent E. coliBL21 Star (DE3) Chemically Competent E. coliBL21 Star (DE3) pLysS Chemically Competent E. coliBL21-AI Chemically Competent E. coli
Recommended for
  • High level expression of non-toxic proteins from low-copy number plasmids
  • Highest expression from low-copy plasmids, enhanced mRNA stability (higher basal expression)
  • Expression from any plasmid, enhanced mRNA stability (lowered basal expression)
  • High-level expression of toxic proteins
Inducing agentIPTGIPTGIPTGIPTGArabinose or Arabinose + IPTG
Protein expression levelHighLower than in BL21 (DE3)Higher than in BL21 (DE3)Lower than in BL21 Star (DE3)High, similar to BL21 (DE3)
Antibiotic resistanceChloramphenicolChloramphenicolTetracycline
Available format(s)
Still not sure which BL21 competent cells are right for your experiment?
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Have any questions on competent cells or transformation? Click on the resources listed below to access overviews, videos, genotype guides, and educational resources.

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