2 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 e ither one of the two SOS-boxes of the Co1E7 operon. The results indica ted that both mutants produced larger amounts of colicin than cells ha rboring the wild-type Co1E7 plasmid. This finding would imply that two biologically functional SOS boxes exist in the Co1E7 operon. In the n on-induced state, no lysis of cells harboring wild-type plasmids occur red at 37 degrees C, whereas, cells harboring recombinant plasmids con taining 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 Co1E operon would obviously tightly control th e expression of Co1E operons. In such a way that it may prevent excess ive expression of the lysis (cel) gene, thus safeguard the host cells from being lysed in ordinary living conditions.