A temporal signal, independent of agr, is required for hla but not spa transcription in Staphylococcus aureus.

Staphylococcus aureus exoprotein expression is controlled by a global regulon known as agr. This system activates transcription of some target genes and represses transcription of others. Target genes expressed postexponentially such as alpha-hemolysin (hla) are activated by agr; target genes expressed during exponential phase such as protein A (spa) are repressed by agr. A unique feature of the agr system is that this transcriptional regulation is mediated by a 517-nucleotide transcript, RNAIII. While it is clear that agr differentially regulates the expression of exponential and postexponential exoproteins, the precise role of agr in the temporal control of these events has not yet been explored. In this report, we examine the effects of expressing RNAIII, the agr regulator, under the control of the inducible beta-lactamase (bla) promoter at different times in the growth cycle. We confirm previous results showing that agr is required for postexponential-phase expression of hla and further show that a separate postexponential-phase signal independent of agr function is also needed for activation of hla transcription. We also show that in an agr mutant transcription of spa occurs throughout the growth cycle, is inhibited immediately upon induction of RNAIII, and is thus indifferent to the postexponential signal required for hla activation.