Two overlapping SOS-boxes in ColE operons are responsible for the viability of cells harboring the Col plasmid.

In this study, oligonucleotide-directed site-specific mutagenesis was used to change the consensus sequences of the LexA binding motifs in either one of the two SOS-boxes of the ColE7 operon. The results indicated that both mutants produced larger amounts of colicin than cells harboring the wild-type ColE7 plasmid. This finding would imply that two biologically functional SOS boxes exist in the ColE7 operon. In the non-induced state, no lysis of cells harboring wild-type plasmids occurred at 37 degrees C, whereas, cells harboring recombinant plasmids containing either one of the mutated SOS boxes underwent lysis within 100 min under the same conditions. This result indicated that adaptation of two SOS boxes of the ColE operon would obviously tightly control the expression of ColE operons. In such a way that it may prevent excessive expression of the lysis (cel) gene, thus safeguard the host cells from being lysed in ordinary living conditions.