Isolation of DNA damage-inducible promoters in Escherichia coli: regulation of polB (dinA), dinG, and dinH by LexA repressor

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Abstract
A new genetic screening method has been developed to isolate Escherichia coli promoters which are components of the SOS regulon. Plasmids containing the regulatory regions of polB (dinA) and two new loci, dinG and dinH, were characterized. Galactokinase gene fusion experiments indicated that transcription of these genes is inducible by treatment with mitomycin and conforms to a classical model of SOS regulation involving simple LexA repression. Mapping studies using the E. coli DNA library of Kohara et al. (Y. Kohara, K. Akiyama, and K. Isono, Cell 50:495-508, 1987) revealed that dinG and dinH are located at 17.8 and 19.8 min on the chromosome, respectively. The nucleotide sequence of the dinH regulatory region contains a segment which is very similar to previously characterized binding sites for LexA protein. An asymmetric, noncanonical 20-bp LexA operator in the cloned dinG promoter region was identified. Additional experiments have revealed that the nucleotide sequence of the gene immediately downstream of the DNA damage-inducible polB locus encodes a polypeptide which has extensive sequence homology to several known and putative DNA and RNA helicase proteins. This gene, which is not regulated by the LexA repressor, has been designated hepA. The predicted amino acid sequence of the product of hepA contains several highly conserved sequence motifs that are also found in enzymes such as the RecQ and UvrB proteins of E. coli and the Rad3 protein of Saccharomyces cerevisiae.