Osmolyte-induced transcription: -35 region elements and recognition by sigma38 (rpoS).

In order to meet osmotic challenges in the gastrointestinal tract, enteric bacteria rapidly accumulate salts of glutamate and other weak organic acids. The ensuing transcriptional activation is mediated by unknown elements at sigma38 (rpoS)-dependent promoters. Here we identify DNA elements needed for high levels of transcription in the presence of salt and acetate and show that they are associated with the -35 regions of target promoters. Unrelated -35 region sequences are shown to specify maximal salt-challenged transcription at the otsB promoter and maximal acetate-challenged transcription at the cfa promoter. Mutants in sigma38 are isolated that contribute to bypassing the salt response and most of these cluster in a small segment corresponding to the presumptive -35 DNA recognition determinant of the protein. Overall, the data suggest that an ensemble of -35 region elements exists at sigma38 promoters and these can help mediate responsiveness to physiological challenges through interactions involving region 4 of the sigma38 protein.