Stl is a master repressor encoded by Staphylococcus aureus pathogenicity islands (SaPIs) that participates in the dissemination of these mobile genetic elements on the bacterial chromosome. After infection or induction of a resident helper phage, the SaPIs are de-repressed by specific interactions of the phage proteins with the Stl repressor, allowing the island to replicate and introduce its genetic material into the phage capsids, taking advantage of its machinery to achieve a horizontal gene transfer when virus particles with genetic material from the island infect other cells. SaPIs have developed a fascinating mechanism to ensure their promiscuous transfer by targeting structurally unrelated proteins that perform identical and conserved functions for the phage, the dimeric and trimeric dUTPases. These proteins would act in a similar way to the proto-oncogenic G proteins, where their moonligthing and signaling function would be regulated by dUTP, which would act as a second messenger, allowing or preventing the binding of the repressor and, therefore, the replication and transfer of the genes of the island. In this thesis we decipher the molecular mechanism of this elegant strategy through mutational assays that, firstly, delimit the areas involved in the interaction and, finally, determining and validating the structure of the Stl repressor of SaPIbov1 alone and in complex with dUTPases from different S. aureus phages. Surprisingly, SaPIbov1 Stl is a modular protein capable of interacting with dimeric and trimeric dUTPases through different domains and mechanisms. In case of the SaPIbov1 Stl complex with trimeric dUTPases, the intermediate domain of the repressor plays a key role through the active center of the enzyme emulating its substrate, dUTP, and interacting with conserved motifs of the same. On the other hand, the formation of the repressor complex with dimeric dUTPases is generated through the Cterminal motif, which not only interacts with the active center of the enzyme but also emulates the dimerization surface of the dUTPase, forming a heterodimer. The adquisition of multispecificity through the recruitment of domains proposes Stl from SaPIbov1 as a new type of repressor, which preserves its mechanism of interaction with DNA but allows an evolution of other parts of the protein with the aim of a wide diffusion of the SaPI proteins.
