Transcriptional crosstalk between helper bacteriophages and Staphylococcal aureus pathogenicity islands

Acquisition of a superantigen pathogenicity island (SaPI) significantly increases virulence in Staphylococcus aureus. Horizontal transfer of SaPIs occurs at high frequency and depends upon a helper bacteriophage, either through direct infection or SOS-mediated induction of a lysogen. SaPIs hijack the packaging machinery of the helper phage, leading to the formation of SaPI-containing transducing particles that can introduce the pathogenicity island into neighboring SaPI-negative cells. All SaPIs contain a conserved core of genes, some of which are co-transcribed as an operon and encode functions involved in helper exploitation. The goal of this study was to more fully understand the intricate relationships between the SaPI elements and their helper bacteriophages, specifically any regulatory crosstalk that might occur between them. We demonstrated phage-host interactions in 80 and 80α, and SaPI1 and SaPIbov1-mediated crosstalk with helper phage 80α. The phage Sri protein was shown to be a bi-functional protein that both derepresses SaPI1 and interferes with host chromosome replication. Incoming SaPI1 experiments showed that SaPI1 modulates the levels of the N-terminal part of orf14 mRNA. Induction experiments using the 80α ΔrinA phage as a genetic tool, reveal several new phage genes that SaPI1 targets for expression modulation. Finally, a novel SaPI1 interference mechanism was identified. In an 80α ΔrinA mutant, which cannot activate its late operon, SaPI1 can directly turn on expression of the packaging and structural genes in a noncanonical manner, initiating from the 2nd gene in the operon, the large terminase subunit