RpoS (sigma(S)) 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 sigma(70) family of proteins. Conserved domain 1.1 at the N-terminal end of sigma(70) 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 sigma(S) variant with a deletion of its N-terminal 50 amino acids (sigma(S)Delta1-50), 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 sigma(S)-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 sigma(S) and cyclic AMP [cAMP]-cAMP receptor protein for its activity) was also preserved in the strain expressing sigma(S)Delta1-50. The intracellular content of sigma(S)Delta1-50 was regulated by culture variables such as growth phase, osmolarity, and temperature in the same manner as that described earlier for sigma(S), 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 sigma(70), the N-terminal domain in sigma(S) may not be essential for the protein to function as a sigma factor in vivo. Furthermore, our results suggest that the induction of sigma(S)-specific promoters in stationary phase and during growth under conditions of high osmolarity or low temperature is mediated primarily through the regulation of sigma(S) protein degradation.