Staphylococcus aureus is an opportunistic pathogen causing many serious human and animal diseases . Infection is associated with the expression of virulence factors , mainly controlled by two regulatory systems agr and sar . Agr locus consists of two transcripts : RNAIII that is RNAII and the regulatory molecule encoding four proteins required for the stimulation of transcription of RNAII and RNAIII . Sar locus encoding a protein SarA and agr activator which binds to DNA is composed of three overlapping reading frames .