Mobile Genetic Element SCCmec-encoded psm-mec RNA Suppresses Translation of agrA and Attenuates MRSA Virulence

Community acquired-methicillin resistant Staphylococcus aureus (CA-MRSA) is a socially problematic pathogen that infects healthy individuals, causing severe disease. CA-MRSA is more virulent than hospital associated-MRSA (HA-MRSA). The underlying mechanism for the high virulence of CA-MRSA is not known. The transcription product of the psm-mec gene, located in the mobile genetic element SCCmec of HA-MRSA, but not CA-MRSA, suppresses the expression of phenol-soluble modulin alpha (PSM alpha), a cytolytic toxin of S. aureus. Here we report that psm-mec RNA inhibits translation of the agrA gene encoding a positive transcription factor for the PSM alpha gene via specific binding to agrA mRNA. Furthermore, 25% of 325 clinical MRSA isolates had a mutation in the psm-mec promoter that attenuated transcription, and 9% of the strains had no psm-mec. In most of these psm-mec-mutated or psm-mec-deleted HA-MRSAs, PSM alpha expression was increased compared with strains carrying intact psm-mec, and some mutated strains produced high amounts of PSM alpha comparable with that of CA-MRSA. Deletion of psm-mec from HA-MRSA strains carrying intact psm-mec increased the expression of AgrA protein and PSM alpha, and virulence in mice. Thus, psm-mec RNA suppresses MRSA virulence via inhibition of agrA translation and the absence of psm-mec function in CA-MRSA causes its high virulence property.