The molecular architecture of the sar locus in Staphylococcus aureus

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Abstract
The global regulator sar in Staphylococcus aureus controls the synthesis of a variety of cell wall and extracellular proteins, many of which are putative virulence factors. The sar locus in strain RN6390 contains a 339-bp open reading frame (sarA) and an 860-bp upstream region. Transcriptional analyses of this locus revealed three different transcripts of 0.58, 0.84, and 1.15 kb (designated sarA, sarC, and sarB, respectively). All three transcripts seemed to be under temporal, growth cycle-dependent regulation, with sarA and sarB being most abundant in early log phase and the sarC concentration being highest toward the late stationary phase. Mapping of the 5' ends of the sar transcripts by primer extension and modified S1 nuclease protection assays demonstrated that transcription is initiated from three separate, widely spaced promoters. The 3' ends of all three sar transcripts are identical, and transcriptional termination occurs upstream of a typical prokaryotic poly(T) termination signal. Northern (RNA) analysis of sar mutant clones containing plasmids that comprised various promoters and the termination signal revealed that individual transcripts can be generated from each of the three promoters, thus suggesting possible activation as independent promoters. The multipromoter system, from which transcription is initiated, bears conserved features for recognition by homologous sigma 70 transcription factors and also by those expressed in the general stress response. Downstream of the two distal promoters (P3 and P2) are two regions potentially encoding short peptides. It is conceivable that posttranslational cooperation between these short peptides and the sarA gene product occurs to modulate sar-related functions. Complementation studies of a sar mutant with a clone expressing all three sar transcripts showed that this clone was able to restore the sar wild-type phenotype to the sar mutant.