Regulation of microcin C51 operon expression: The role of global regulators of transcription

Expression of the microcin C51 operon in Escherichia coli cells is regulated as a function of the phase of growth; it is stimulated during the decelerating phase of growth. Using single-copy P_mcc-lac transcriptional fusion (the promoter region of the microcin C51 operon fused to a promoterless lac operon in λ phage), we showed that transcription from the microcin operon promoter is dependent on σ^s (RpoS) factor. However, some level of P_mcc-lac expression is possible in rpoS null mutants, indicating that another sigma factor might be involved in transcription of the microcin C51 operon. Overproduction of σ⁷⁰ decreased P_mcc-directed transcription, presumably as a result of competition of sigma factors for the limited amount of core RNA polymerase. The cyclic AMP-CRP complex was shown to stimulate transcription from P_mcc: the absence of CRP or cAMP in crp or cya mutant cells strongly decreased the level of P_mcc-lac expression. The production of C51 microcin decreased or was absent in rpoS, crp and cya mutant cells. Leucine-responsive protein Lrp and histone-like protein H-NS repressed P_mcc-lac expression in the exponential and decelerating phases of growth. In studies of P_mcc-lac expression in double mutant cells, we showed that proteins CRP, Lrp and H-NS acted in rpoS-dependent and rpoS-independent ways in transcription of the microcin C51 operon. Mutation hns^- resulted in an increase in P_mcc-lac expression in crp, rpoS and lrp mutant cells, as in wild-type cells.