Influence of laterally acquired genetic elements on the physiology of Salmonella enterica

INFLUENCE OF LATERALLY ACQUIRED GENETIC ELEMENTS ON THE PHYSIOLOGY OF SALMONELLA ENTERICA Salmonella spp. are accountable for a large fraction of the global infectious disease burden, with most of their infections being food- or water-borne. As the phenotypic features and adaptive potential of Salmonella spp. appear to be driven mainly by mobile or laterally acquired genetic elements, a better understanding of the behaviour and diversification of these important pathogens consequently requires a more profound insight into the different mechanisms by which these pivotal elements can affect cellular physiology. In this work, research was accordingly focused on the impact of a laterally acquired endonuclease (i.e. Mrr), and a temperate bacteriophage (i.e. P22) on the physiology of Salmonella Typhimurium LT2. Mrr is a cryptic Type IV restriction endonuclease that is characterized by its specificity for modified DNA. The gene encoding Mrr is located in the immigration control region , a foreign genetic element that harbours a large number of restriction systems. In contrast to the Mrr protein of its close relative, Escherichia coli MG1655, we initially found Mrr of LT2 to be inactive. Closer analysis, however, revealed that degeneration of LT2 Mrr might have been enforced by the presence of a laterally acquired Type III methyltransferase in LT2, whose activity in the cell is incompatible with a functional Mrr enzyme. Nevertheless, random mutagenesis revealed that a number of seemingly unrelated mutations could each still render LT2 Mrr constitutively active, indicating its degeneration is readily reversible. P22, on the other hand, is a temperate and Salmonella-specific model phage that over the last 50 years has greatly contributed to our understanding of phage biology and phage host interactions in general. Using a random promoter-trap library, we stumbled upon a novel interaction between P22 and LT2, which is characterized by the deliberate redirection of the host s metabolism. Interestingly, we could identify the actual instigator of this interaction as a small ORFan protein encoded on the P22 genome, but its deletion did not obviously affect infection or lysogenisation by P22. In summary, the research in this work revealed a number of novel mechanisms by which specific mobile or laterally acquired genetic elements can determine or interfere with the behaviour of S. Typhimurium.