SarA is a repressor of hla (alpha-hemolysin) transcription in Staphylococcus aureus: its apparent role as an activator of hla in the prototype strain NCTC 8325 depends on reduced expression of sarS.

In most Staphylococcus aureus strains, inactivation of sarA increases hla transcription, indicating that sarA is a repressor. However, in S. aureus NCTC 8325 and its derivatives, used for most studies of hla regulation, inactivation of sarA resulted in decreased hla transcription. The disparate phenotype of strain NCTC 8325 seems to be associated with its rsbU mutation, which leads to sigma(B) deficiency. This has now been verified by the demonstration that sarA repressed hla transcription in an rsbU+ derivative of strain 8325-4 (SH1000). That sarA could act as a repressor of hla in an 8325-4 background was confirmed by the observation that inactivation of sarA in an agr sarS rot triple mutant dramatically increased hla transcription to wild-type levels. However, the apparent role of sarA as an activator of hla in 8325-4 was not a result of the rsbU mutation alone, as inactivation of sarA in another rsbU mutant, strain V8, led to increased hla transcription. Northern blot analysis revealed much higher levels of sarS mRNA in strain V8 than in 8325-4, which was likely due to the mutation in the sarS activator, tcaR, in 8325-4, which was not found in strain V8. On the other hand, the relative increase in sarS transcription upon the inactivation of sarA was 15-fold higher in 8325-4 than in strain V8. Because of this, inactivation of sarA in 8325-4 means a net increase in repressor activity, whereas in strain V8, inactivation of sarA means a net decrease in repressor activity and, therefore, enhanced hla transcription.