rRNA regulation during growth and under stringent conditions in Staphylococcus aureus

The control of rRNA synthesis and, thereby, translation is vital for adapting to changing environmental conditions. The decrease of rRNA is a common feature of the stringent response, which is elicited by the rapid synthesis of (p)ppGpp. Here we analysed the properties and regulation of one representative rRNA operon of Staphylococcus aureus under stringent conditions and during growth. The promoters, P1 and P2, are severely downregulated at low intracellular guanosine triphosphate (GTP) concentrations either imposed by stringent conditions or in a guanine auxotroph guaBA mutant. In a (p)ppGpp(0) strain, the GTP level increased under stringent conditions, and rRNA transcription was upregulated. The correlation of the intracellular GTP levels and rRNA promoter activity could be linked to GTP nucleotides in the initiation region of both promoters at positions between +1 and +4. This indicates that not only transcriptional initiation, but also the first steps of elongation, requires high concentrations of free nucleotides. However, the severe downregulation of rRNA in post-exponential growth phase is independent of (p)ppGpp, the composition of the initiation region and the intracellular nucleotide pool. In summary, rRNA transcription in S. aureus is only partially and presumably indirectly controlled by (p)ppGpp.