Bacteria live in many diverse environments and have evolved several strategies to inhibit the growth of neighboring microbe competitors. One such system that has been recently characterized is Contact-Dependent Inhibition (CDI). CDI is a type V secretion system encoded by a three gene locus consisting of cdiB, cdiA, and cdiI, and is widespread amongst alpha, beta, and gamma probacteria CdiB acts as the outer-membrane export channel for CdiA, which extends outward from the surface of the cell. CdiA contains a C-terminal toxin domain (CdiA-CT) that is translocated into neighboring cells upon contact, resulting in growth inhibition. CdiI is an immunity peptide for the toxic domain of CdiA and prevents autoinhibition of the cell. CdiA is highly conserved within a given bacterial species, but is highly polymorphic at the C-terminus. CdiI is also polymorphic, covarying with the CdiA-CT. Together, the CdiA-CT and CdiI proteins form functional and highly specific toxin/immunity pairs that allow CDI+ bacteria to inhibit the growth of neighbors and compete in an environmental niche.

This thesis addresses several of the specific biochemical activities of the CdiA-CT toxins present in different bacterial species in order to provide an explanation of how CDI inhibits target cell growth. Chapter 1 describes various systems which are utilized by bacteria to compete with neighboring cells, including colicins and type III-VI secretion systems. In addition, a summary of what is currently known about the CDI system is presented, including the delivery pathway and previously characterized toxin activities in order to differentiate this system from other modes of bacterial competition. Chapter 2 focuses on the biochemical activities of several toxins that are found in environmental strains of Burkholderia pseudomallei. Chapter 3 provides a comparison study between two distinct toxin/immunity pairs from different species (B. pseudomallei and E. coli), including structural and biochemical comparisons. In Chapter 4 a separate contact-dependent inhibition system encoded by the Rhs proteins of Dickeya dadantii is discussed. I show that these Rhs systems are responsible for the delivery of Rhs-CT effectors into target cells, and that these toxins exhibit DNase activity. Chapter 5 provides a summary of the results presented in the previous chapters, and outlines several of the questions remaining about the mechanisms surrounding CDI and Rhs toxin delivery systems. We believe that this research contributes to the biochemical knowledge of how toxin domains inhibit cell growth.