Negative regulation of σ70-driven promoters by σ70

The Escherichia coli yjbEFGH operon, encoding genes involved in exopolysaccharide production, has previously been shown to be induced by osmotic stress and to be negatively regulated by σ38. Promoter analysis suggested that like most E. coli genes, its transcription is driven by the housekeeping sigma factor σ70. Indeed, manipulation of any of the other five alternative sigma factors did not affect its induction by osmotic stress. Surprisingly, when assayed in a strain expressing low levels of σ70, yjbEFGH induction in response to osmotic stress was higher than in a strain expressing normal levels of σ70. Similar phenomena were observed in the σ70-driven promoters of sulA, uvrA, recA, fecI, entC and lacZ, the transcription of which is directly controlled by a repressor protein (LexA, Fur and LacI), but not in promoters of the housekeeping genes ftsA and ftsY, or in σ38-driven treA promoter. Since transcription factors are generally present in the cell in low numbers, we hypothesize that a decrease in σ70, that drives the expression of lexA, fur and lacI as well, further diminishes their number in the cell and thus de-represses the induction of genes which are subjected to their repression.