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The alkaline shock protein Asp23 was identified as a σB-dependent protein in Staphylococcus aureus. In Bacillus subtilis, the asp23 promoter from S. aureus is regulated like other σB-dependent promoters, which are strongly induced by heat and ethanol stress. However, almost no induction of asp23 expression was found after heat or ethanol stress in S. aureus MA13 grown in a synthetic medium, where the basal expression level of asp23 is high. Under the same experimental conditions the σ B gene itself showed a similar expression pattern: it was highly expressed in synthetic medium but not induced by heat or ethanol stress. In contrast, σB activity was increased by heat stress when the cells were grown in a complex medium. The constitutive expression of sigB and σB-dependent stress genes in S. aureus MA13 grown in a synthetic medium is in a sharp contrast to the regulation of σB activity in B. subtilis, and needs further investigation. A deletion of 11 bp in the rsbU gene, which encodes the phosphatase that acts on RsbV (the anti-anti-sigma factor), in S. aureus NCTC 8325-4 might be responsible for the failure of heat stress to activate σB in complex medium, and thus reduce the initiation of transcription at σB-dependent promoters in this strain. [ABSTRACT FROM AUTHOR]