RNAIII is a stable 514 nt regulatory RNA transcribed by the P3 promoter of the Staphylococcus aureus quorum-sensing agr system [1][2]).
It is the major effector of the agr regulon, which controls the expression of many S. aureus genes encoding exoproteins and cell wall associated proteins plus others encoding regulatory proteins [3][4] The RNAIII transcript also encodes the 26 amino acid δ-haemolysin (Staphylococcus aureus delta toxin) peptide (Hld).
[5] RNAIII contains many stem loops, most of which match the Shine-Dalgarno sequence involved in translation initiation of the regulated genes.
[6][7] In vitro, RNAIII is expressed post exponentially, inhibiting translation of the surface proteins, notably protein A, while stimulating that of the exoproteins, many of which are tissue-degrading enzymes or cytolysins.
[8] Among the latter is the important virulence factor, α-hemolysin (or alpha toxin) (Hla), whose translation RNAIII activates by preventing the formation of an inhibitory foldback loop in the hla mRNA leader.