Reduced transcription of the rnh gene in Escherichia coli mutants expressing the SOS regulon constitutively.

We have analysed the transcription levels for the convergently overlapping Escherichia coli genes for the DNA polymerase III proofreading function (dnaQ) and ribonuclease H (rnh). The two tandem dnaQ promoters are about three times more active than the single rnh promoter as shown by analysing the level of in vivo transcription using dnaQ-galK and rnh-galK fusions. In E. coli mutants constitutively expressing the pleiotropic SOS response, which includes activities that enhance DNA repair, recombination and mutagenesis, a strong reduction in rnh transcription was observed. The lexA51 recA441 double mutant which fully expresses the SOS response shows the strongest reduction in rnh transcription and the highest increase in dnaQ transcription. Nuclease S1 mapping supported the finding that a constitutive expression of SOS function leads to a strong reduction in rnh transcription.