Studies on Regulation of the Colicin El Gene Expression Using Promoter－operator Mutants

Promoter-operator mutations of the colicin El gene on ColEl plasmid of Escherichia coli were isolated using site-directed chemical mutagenesis. The operator region of the gene is constituted by two overlapped SOS boxes which has high affinity to LexA repressor. DNA sequence analysis of the mutants revealed that base changes were in both the SOS boxes as well as the downstream region. Studies on gene expression in vivo and in vitro suggest the followings : 1) multiple point mutations within the terminal trinucleotide consensus sequences of the SOS bowes are necessary for derepression of the colicin El gene, 2) the intermediary portion between terminal consensus sequences has no significant effect, 3) the Pribnow boxes which are created by base substitutions shift the transcription initiation sites towards downstream, and 4) the downstream sequence together with the central overlapping position of the trinucleotide consensus sequense may be important in the LexA repressorDNA interaction.

Bacterial toxin

SOS response

Localized mutagensis

Gene expression

DNA sequencing