Pseudomonas syringae is a pathogen of numerous plant species, including several economically important crops. P. syringae pv. syringae B728a is a resident on leaves of common bean, where it utilizes several well-studied virulence factors, including secreted effectors and toxins, to develop a pathogenic interaction with its host. The B728a genome was recently sequenced, revealing the presence of 1,297 genes with unknown function. This dissertation demonstrates that a 29.9-kb cluster of genes in the B728a genome encodes a novel secretion pathway, the type VI secretion system (T6SS), that functions to deliver at least one protein outside of the bacterial cell. Western blot analyses show that this secretion is dependent on clpV, a gene that likely encodes an AAA + ATPase, and is repressed by retS, which apparently encodes a hybrid sensor kinase. RetS and a similar protein called LadS are shown to collectively modulate several virulence-related activities in addition to the T6SS. Plate assays demonstrate that RetS negatively controls mucoidy, while LadS negatively regulates swarming motility. A mutation in retS affects B728a population levels on the surface of bean leaves. A model for the LadS and RetS control of B728a virulence activities is proposed, and possible roles for the B728a T6SS are addressed.
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