The protein encoded by katF (also known as nur, appR, csi-2, abrD, and rpoS in various alleles) has been biochemically confirmed to be an alternate sigma transcription factor and renamed sigma S. Its synthesis is controlled transcriptionally and posttranscriptionally by as yet undefined mechanisms that are active well into stationary phase. sigma S controls a regulon of 30 or more genes expressed in response to starvation and during the transition to stationary phase. Proteins in the regulon, many of which have not been characterized, enhance long-term survival in nutrient-deficient medium and have a diverse group of functions including protection against DNA damage, the determination of morphological changes, the mediation of virulence, osmoprotection, and thermotolerance. Differential expression of subfamilies of genes within the regulon is effected by supplementary regulatory factors, working both individually and in combination to modulate activity of different sigma S-dependent promoters.
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