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Bacteria, both pathogenic and non-pathogenic, have developed multiple forms of competition mechanisms to combat each other including, but not limited to, Contact-Dependent growth Inhibition (CDI) systems, Type VI Secretion Systems and the associated Rearrangement hotspot (Rhs) toxin system. These systems usually confers a great fitness advantage as they allow for precise delivery of toxic molecules into competing bacteria whilst sister cells are protected from auto-inhibition by producing a cognate immunity protein. Delivery between sister cells may serve as a form of “self-recognition” whilst maintaining selection pressure for these genes within the population. How these genes are maintained in conditions where delivery does not occur has until now not been fully understood.

This thesis describes secondary functions, maintained selection pressure and regulation of Rhs and CDI systems in three parts. In Paper I, we made a novel discovery that rhs toxin and immunity genes from Salmonella enterica serovar Typhimurium are transcribed from internal transcriptional start sites independent of the full length delivery gene. This results in functional cytosolic proteins capable of regulating proliferation and growth rate of S. Typhimurium during infection of RAW264.7 cells. In Paper II, we continued our work from Paper I and studied growth effects in vitro as well as regulation of the internal expression. Our findings show that Rhs causes a small fitness cost also in rich laboratory medium and is regulated by alternate sigma factor RpoS, two-component system PhoP/Q and DNA binding protein H-NS. In Paper III, we made a discovery similar to our findings in Paper I and II by observing internally transcribed toxin and immunity genes of  multiple CDI systems from E. coli regulated by RpoS. We propose that CDI toxin-immunity pairs function as selfish genetic elements that maintain gene selection whilst simultaneously retaining the ability to protect the cell from externally delivered toxins.