What's up, Dut? Help, help I'm being derepressed; the great derepression of SaPIs or how I learned to stop worrying and love mobilization

Staphylococcus aureus is known to cause disease in both human and animal populations. Many of the virulence factors important for S. aureus pathogenesis are found on mobile genetic elements known as S. aureus pathogenicity islands (SaPIs). SaPIs can transduce into surrounding cells, giving those cells the ability to produce virulence factors. Derepression allows SaPIs to be excised from S. aureus genome to initiate mobilization. Derepressors have been identified for multiple SaPIs, but it remains unknown if the same SaPI can be derepressed by multiple derepressors. It has been shown that Stl_bov1 can be derepressed by the type 1 dUTPase from 80α and &phis;11 helper phages through interaction with Stl_bov1 Stl (Stl_bov1), the SaPI repressor. We previously noted that &phis;NM1 can transduce Stl_bov1, although it lacks a type 1 dUTPase. The purpose of this study was to determine if Stl_bov1 could be derepressed by an analogous protein, and if it has similar interactions with Stl_bov1 as type 1 dUTPase. We identified the derepressor as a type 2 dUTPase encoded by &phis;NM1 (Dut_NM1), which interacted directly with Stl_bov1. This interaction causes release of the promoter needed for expression of excisionase and integrase. Even though the structures of type 1 and type 2 dUTPases are completely different, they are still able to interact with Stl_bov1. Stl_bov1 interaction with Dut_NM1 inhibits enzyme activity. Since the ability to hydrolyze dUTP is a common function to the two different types of dUTPases, we hypothesized that it was integral to the interaction with Stl_bov1 . Interestingly, the inactive mutants show this is not the case, because they are able to interact with Stl_bov1. Only the null activity mutant lacking all hypothetical Mg²⁺ binding residues of Dut _NM1 fails to cause Stl_bov1 to release the promoter. It is possible that the Mg²⁺ binding or those conserved residues of Dut_NM1 are important for Stl to release the promoter. This work has thus identified a novel derepressor and so provides an enhanced understanding of the derepression mechanisms of SaPIs.

Keywords: Staphylococcus aureus pathogenicity islands, horizontal gene transfer, derepression, dUTPase, Stl, protein interactions