Characterization of the Type VI Secretion System in Enterobacter cloacae

Bacteria live in complex communities and must compete with their neighborsfor resources. They have therefore evolved multiple different competition systems inorder to improve their fitness in dense, complex environments. The type VI secretionsystem (T6SS) is a Gram-negative weapon used to inhibit the growth of neighboringcells. It does this by delivering toxic effector proteins directly into neighbors using aspeargun-like apparatus. A spear-shaped protein complex is propelled out of theT6SS-expressing cell, and this spear punctures a neighboring cell to deliver its toxicpayload. Toxic effector proteins are bound either covalently or non-covalently to thissecreted complex, and these toxins can target both periplasmic or cytoplasmicsubstrates, as well as membranes. Kin protect themselves from these effectors byexpressing cognate immunity proteins to block effector activity.This thesis explores several aspects of the T6SS in Enterobacter cloacae ATCC13047. In Chapter 1, I provide a general introduction to the T6SS and covers topicssuch as the structure and assembly of the apparatus, regulation of T6SS loci andeffectors, and effector diversity. I then describe the genetics of the T6SS in E. cloacaeand explore what effectors are deployed in this system in Chapter 2. In Chapter 3, Ifocus on one particular effector deployed by this system, the type VI secretion systemlipase effector, Tle, and investigate its intriguing reliance on its cognate immunityprotein, Tli, for toxicity. Next, Chapter 4 discusses the role of rearrangement hotspotix(Rhs) proteins in the assembly of the T6SS apparatus. Chapter 5 then looks at therole of the Rhs accessory protein effector-associated gene with Rhs (EagR). Finally, Isummarize my findings and discuss open questions in the field in Chapter 6.