SarT, a repressor of alpha-hemolysin in Staphylococcus aureus

In searching the Staphylococcus aureus genome, we found several homologs to SarA. One of these genes, sarT, codes for a basic protein with 118 residue s and a predicted molecular size of 16,096 Da. Northern blot analysis revea led that the expression of sarT was repressed by sarA and agr. An insertion sarT mutant generated in S. aureus RN6390 and 8325-4 backgrounds revealed minimal effect on the expression of sarR and sarA. The RNAIII level was not ably increased in the sarT mutant, particularly in postexponential-phase ce lls, while the augmentative effect on RNAII was less. SarT repressed the ex pression of a-hemolysin, as determined by Northern blotting, Western blotti ng, and a rabbit erythrocyte hemolytic assay. This repression was relieved upon complementation. Similar to agr and sarA mutants, which predictably di splayed a reduction in hla expression, the agr sarT mutant exhibited a lowe r level of hla transcription than the sarT mutant. In contrast, hla transcr iption was enhanced in the sarA sarT mutant compared with the single sarA m utant. Collectively, these results indicated that the sarA locus, contrary to the regulatory action of agr, induced a-hemolysin production by repressi ng sarT, a repressor of hla transcription.