Figure 2. Differences in the promoter sequences that control the transcription of the T7 RNA polymerase gene between E. coli BL21(DE3) and fifteen mutant derivatives that overcame the transformation toxicity induced by a membrane protein-encoding gene.

Three strains in the mutant group BL1 had the same promoter sequence as C41(DE3) and C43(DE3). Nine BL2 mutants had reverted the lacUV5 promoter sequence to that of the wild-type lac promoter. Three BL3 strains showed no difference from the lacUV5 promoter of BL21(DE3).