Involvement of Sigma S and Sigma 70 Subunits of RNA Polymerase and the CRP Protein in the Regulation of Microcin C51 Operon Expression

Abstract

Expression of the microcin C51 operon in Escherichia coli cells is activated during cell entry into the stationary growth phase and depends on the sigmaS subunit of RNA polymerase (RpoS). The null rpoS mutations retained the residual expression level of the transcriptional P(mcc)-lac fusion, which indicates that other sigma subunit can participate in the regulation of transcription of the microcin C51 operon. Data presented in this work show that the overproduction of sigma70 in rpoS- cells diminished the level of P(mcc)-lac expression, as in wild-type cells, which seems to be the consequence of competition between sigma factors for a limited number of core RNA polymerase molecules. In the presence of the rpoD800 mutation that renders sigma70 temperature-sensitive, expression of P(mcc)-lac was not induced in the phase of delayed culture growth at nonpermissive temperature, which indicates that sigma70 is indispensable for microcin operon expression. Point substitutions in the -10 P(mcc) region, leading to the formation of 5'-TGaTATAAT-3' site, enhanced promoter activity but did not affect the relationship between P(mcc)-lac transcription and growth phase, sigmaS, and the activator protein CRP. The activator protein CRP was shown to bind a DNA fragment containing the TGTGA(AATGAA)TCTAT site in the -59.5 bp position relative to the start site of transcription. Mutation in the ssrI gene encoding 6S RNA did not disturb P9mcc)-lac expression; these results indicate that 6S RNA does not participate in the regulation of microcin C51 operon expression.