UC Santa Barbara

Bacteria live in complex communities and must compete with their neighbors

for resources. They have therefore evolved multiple different competition systems in

order to improve their fitness in dense, complex environments. The type VI secretion

system (T6SS) is a Gram-negative weapon used to inhibit the growth of neighboring

cells. It does this by delivering toxic effector proteins directly into neighbors using a

speargun-like apparatus. A spear-shaped protein complex is propelled out of the

T6SS-expressing cell, and this spear punctures a neighboring cell to deliver its toxic

payload. Toxic effector proteins are bound either covalently or non-covalently to this

secreted complex, and these toxins can target both periplasmic or cytoplasmic

substrates, as well as membranes. Kin protect themselves from these effectors by

expressing cognate immunity proteins to block effector activity.

This thesis explores several aspects of the T6SS in Enterobacter cloacae ATCC

13047. In Chapter 1, I provide a general introduction to the T6SS and covers topics

such as the structure and assembly of the apparatus, regulation of T6SS loci and

effectors, and effector diversity. I then describe the genetics of the T6SS in E. cloacae

and explore what effectors are deployed in this system in Chapter 2. In Chapter 3, I

focus on one particular effector deployed by this system, the type VI secretion system

lipase effector, Tle, and investigate its intriguing reliance on its cognate immunity

protein, 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 the

role of the Rhs accessory protein effector-associated gene with Rhs (EagR). Finally, I

summarize my findings and discuss open questions in the field in Chapter 6.