High-level expression of heterologous proteins
Carsten-Peter Carstens Julie Bonnardel Anna Waesche
Codon bias is a significant obstacle for efficient expression of heterologous genes in E. coli hosts. In GC-rich genomes, such as mammals, rare arginine codons (AGG or AGA) and the proline codon (CCC) most frequently affect bacterial gene expression. For such situations, Stratagene introduces BL21-CodonPlus-RP cells* to make protein expression more reliable. This strain contains a ColE1-compatible plasmid that encodes extra copies of the argU and proL tRNA genes and is able to rescue expression of genes restricted by either AGG/ AGA codons or CCC codons.
The E. coli BL21 strains offer many advantages for expression of heterologous proteins. Derived from E. coli B, these strains naturally lack the Lon protease and are engineered to be deficient for the OmpT protease. The BL21(DE3) and the BL21(DE3)pLysS hosts permit induced expression of T7 RNA polymerase. When used in conjunction with pET-type vectors, the expressed T7 RNA polymerase provides significant levels of transcription of heterologous genes. Stratagenes BL21-Gold competent cells** are another member of the BL21 family of cells that feature two additional advantages: They are EndA1 deficient, which allows preparation of high-quality miniprep DNA, and they have the Hte phenotype, which confers increased transformation efficiency.1
Even with these many innovations, the expression of heterologous proteins in E.
coli can be difficult. Most frequently, failed or insufficient expression of
the heterologous gene is caused by the presence of codons that are rarely used
in E. coli.2,3 Forced high-level expression of genes with rare
codons for the E. coli host can lead to depletion of the endogenous pools
of the corresponding tRNAs.