Hyperproduction of alpha-hemolysin in a sigB mutant is associated with elevated SarA expression in Staphylococcus aureus

To evaluate the role of SigB in modulating the expression of virulence dete rminants in Staphylococcus aureus, we constructed a sigB mutant of RN6390, a prototypic S. aureus strain. The mutation in the sigB gene was confirmed by the absence of the SigB protein in the mutant on an immunoblot as well a s the failure of the mutant to activate sigma B-dependent promoters (e.g., the sarC promoter) of S. aureus. Phenotypic analysis indicated that both al pha-hemolysin level and fibrinogen-binding capacity were up-regulated in th e mutant strain compared with the parental strain. The increase in fibrinog en-binding capacity correlated with enhanced expression of clumping factor and coagulase on immunoblots. The effect of the sigB mutation on the enhanc ed expression of the alpha-hemolysin gene (hla) was primarily transcription al. Upon complementation with a plasmid containing the sigB gene, hla expre ssion returned to near parental levels in the mutant. Detailed immunoblot a nalysis as well as a competitive enzyme-linked immunosorbent assay of the c ell extract of the sigB mutant with anti-SarA monoclonal antibody 1D1 revea led that the expression of SarA was higher in the mutant than in the parent al control. Despite an elevated SarA level, the transcription of RNAII and RNAIII of the agr locus remained unaltered in the sigB mutant. Because of a lack of perturbation in agr, we hypothesize that inactivation of sigB lead s to increased expression of SarA which, in turn, modulates target genes vi a an agr-independent but SarA-dependent pathway.