Competent Cells for Protein Expression | Thermo Fisher Scientific

A person in a laboratory wearing a mask and gloves and holding a petri dish

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

Product
Product	BL21 (DE3) Chemically Competent E. coli	BL21(DE3)pLysS Chemically Competent E. coli	BL21 Star (DE3) Chemically Competent E. coli	BL21 Star (DE3) pLysS Chemically Competent E. coli	BL21-AI Chemically Competent E. coli
Recommended for	High level expression of non-toxic proteins from low-copy number plasmids	Low basal expression and high induced protein expression For even further reduced basal expression, see BL21(DE3)pLysE Chemically Competent E. coli	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 agent	IPTG	IPTG	IPTG	IPTG	Arabinose or Arabinose + IPTG
Protein expression level	High	Lower than in BL21 (DE3)	Higher than in BL21 (DE3)	Lower than in BL21 Star (DE3)	High, similar to BL21 (DE3)
Antibiotic resistance	–	Chloramphenicol	–	Chloramphenicol	Tetracycline
Available format(s)	21 x 50 µL/tube	11 x 50 µL/tube 21 x 50 µL/tube	21 x 50 µL/tube 1 x 96-tube rack with 50 µL/tube	21 x 50 µL/tube	21 x 50 µL/tube

Still not sure which BL21 competent cells are right for your experiment?

Compare genotypes of BL21 strains
Shop all Invitrogen BL21 competent cells
Explore other tools for bacterial protein expression

Competent cells: Featured products

In addition to competent cells, we offer a broad suite of products for the cloning workflow.

Magnified image of purple, rod-shaped bacteria

Microbial
media

Explore types of bacterial growth media to use for cloning, plasmid DNA preparation, and protein expression.

Browse media

Electroporation
cuvettes

Learn more about universal-fit electroporation cuvettes for use with competent cells.

View electroporation cuvettes

Photo of TOPO PCR cloning kit product box and content

TOPO PCR
cloning kits

Learn more about one-step PCR cloning that combines traditional restriction and ligation cloning.

View TOPO kits

Gibson Assembly
cloning kits

Gibson Assembly cloning kits provide highly efficient, seamless cloning, enabling the assembly of multiple DNA fragments of varying lengths into any vector.

Explore Gibson Assembly cloning

Resources

Have any questions on competent cells or transformation? Click on the resources listed below to access overviews, videos, genotype guides, and educational resources.