ABSTRACT
RpoS (σS) in Escherichia coli is a stationary-phase-specific primary sigma factor of RNA polymerase which is 330 amino acids long and belongs to the eubacterial σ70 family of proteins. Conserved domain 1.1 at the N-terminal end of σ70 has been shown to be essential for RNA polymerase function, and its deletion has been shown to result in a dominant-lethal phenotype. We now report that a σS variant with a deletion of its N-terminal 50 amino acids (σSΔ1-50), when expressed in vivo either from a chromosomal rpoS::IS10 allele (in rho mutant strains) or from a plasmid-borne arabinose-inducible promoter, is as proficient as the wild type in directing transcription from the proU P1 promoter; at three other σS-dependent promoters that were tested (osmY, katE, and csiD), the truncated protein exhibited a three- to sevenfold reduced range of activities. Catabolite repression at the csiD promoter (which requires both σS and cyclic AMP [cAMP]-cAMP receptor protein for its activity) was also preserved in the strain expressing σSΔ1-50. The intracellular content of σSΔ1-50 was regulated by culture variables such as growth phase, osmolarity, and temperature in the same manner as that described earlier for σS, even when the truncated protein was expressed from a template that possessed neither the transcriptional nor the translational control elements of wild-type rpoS. Our results indicate that, unlike that in σ70, the N-terminal domain in σS may not be essential for the protein to function as a sigma factor in vivo. Furthermore, our results suggest that the induction of σS-specific promoters in stationary phase and during growth under conditions of high osmolarity or low temperature is mediated primarily through the regulation of σS protein degradation.
Correction: Heterogeneity of the Principal Factor in Escherichia coli: The rpoS Gene Product, 38, is a Second Principal Factor of RNA Polymerase in Stationary-Phase Escherichia coli