In the human pathogen Staphylococcus aureus, many proteins involved in the infection process are preferentially produced during the stationary growth phase. Using a DNA probe corresponding to the Bacillus subtilis gene encoding the stationary-phase sigma factor SigB (sigmaB), we identified a gene in S. aureus with similarity to B. subtilis sigB. The sigB region was mapped on the SmaI I fragment of the S. aureus chromosome and contains a total of six open reading frames (orf1-6). The deduced amino acid sequences of orf2, orf3, orf4, and orf5 show 64, 67, 71, and 77% similarity to the B. subtilis proteins RsbU, RsbV, RsbW, and SigB, respectively, with SigB representing the sigma factor and the Rsb proteins representing regulators of sigma B. Furthermore, the relative position of the corresponding genes is conserved in B. subtilis, which strongly suggests that we identified the sigB operon of S. aureus, encoding an alternative sigma factor in this organism. The proposed gene products of the two remaining open reading frames show 48-62% similarity to the PemK, ChpAK, and ChpBK growth inhibitors of Escherichia coli (ORF1) and 61% similarity to the ribosomal protein S1 of Haemophilus influenzae (ORF6). Northern blot analysis of the sigB region in S. aureus revealed that four different transcripts are expressed under different conditions of growth phase and stress. These results indicate a complex transcriptional regulation that differs between S. aureus and B. subtilis.