Both the C41(DE3) and C43(DE3) strains are derivatives of the safety level 1 E. coli B strain, BL21(DE3).
The uncharacterized mutations of C41(DE3) and C43(DE3) occurred by a spontaneous process, followed by a phenotypic selection based on the ability to survive after inducing the expression of a toxic recombinant protein (1).
Like the parental BL21(DE3) strain, C41(DE3) and C43(DE3) carry the lambda DE3 lysogen which expresses T7 RNA polymerase from the lacUV5 promoter by Isopropyl-1-thio-ß-D-galactopyranoside (IPTG) induction.