title: The SaPI master repressor and the bacteriophage Duts: a tale of parasitism, evolution, and interspecific transfer
Creators: name: Bowring, Janine Zara
Publisher: name: University of Glasgow
Year: 2018
Subjects: subject: SaPI, Dut, dUTPase, Staphylococcus aureus, pathogenicity islands, Stl repressor, bacteriophage.

description: Staphylococcus aureus pathogenicity islands (SaPIs) form part of the wider family of phage inducible chromosomal islands (PICIs), which are extremely mobile phage satellites. SaPIs are clinically relevant, encoding and disseminating virulence and fitness factors to promote bacterial evolution and pathogenesis. These islands are innately linked to their helper phage’s life cycle, requiring phage-mediated induction and packaging for transfer between bacterial hosts. The inherent association between SaPIs and helper phages is one of the best characterised examples of virus satellite interactions in prokaryotic cells. The SaPIs sit passively in the host chromosome, controlled by the SaPI-encoded master repressor protein; the Stl. Interaction between the SaPIs and their helper phages begins when a phage-encoded protein binds to the Stl to de-repress the island. Different SaPIs encode different Stl repressors, which require diverse phage-encoded proteins for derepression. The best characterised phage-encoded inducers are the trimeric dUTPases (Duts), which traditionally prevent the misincorporation of uracil into DNA, but have been proposed as regulatory proteins that induce SaPIbov1. Recently, the structurally dissimilar phage-encoded dimeric Dut of NM1 has also been shown to mobilize SaPIbov1, but how this is accomplished remains unsolved. Here, this work expands the family of SaPIbov1 phage-encoded inducers to include the subset of phage-encoded dimeric Duts. This develops upon the theory that specific phage-proteins are required for SaPI induction, broadening this vision to show that SaPIs instead target conserved phage mechanisms. Using SaPIbov1 as an example, this study indicates the StlSaPIbov1 can interact with the structurally distinct dimeric and trimeric Duts, whose only shared value is their role for the phage. Furthermore, this study highlights that, although these two proteins are completely unrelated, they both encode one highly variable region (named motif VI). Indeed, the mechanism for the dimeric Dut interaction with the StlSaPIbov1 is investigated here and a number of parallels to the trimeric Dut mode of action are apparent. These results indicate that the structurally divergent dimeric and trimeric Duts interact with the SaPIbov1 Stl using analogous, but distinct mechanisms that represent a fascinating example of convergent evolution. Furthermore, investigations into the SaPIbov1 Stl indicate that Stl accomplishes these interactions with the dimeric and trimeric Duts by using separate domains for each interaction. This intriguing Stl evolution is an elegant strategy utilised by the SaPIs to overcome phage-encoded inducer mutation or exchange. This highlights the continual arms race between the phages, which aim to escape SaPI induction, and the SaPIs, which must overcome these changes for island induction to occur. Furthermore, these results highlight the potential for Stl targeting of structurally diverse, functionally related phage proteins to facilitate intra- and inter-specific transfer. Overall, these results highlight the SaPIs as a fascinating subcellular parasite of the phages, which have evolved a novel strategy to target their phage inducer proteins and achieve transfer. Likewise, this study shows the biological significance of the dimeric Duts as regulatory molecules.