Jeremiah Marden
Graduate Student

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Rhodospirillum centenum is an anoxygenic photosynthetic bacterium that forms metabolically inactive, desiccation resistant cysts in response to nutrient deprivation, somewhat analogous to myxospore formation in Myxococcus. While much is known of the mechanisms governing sporulation, those that regulate bacterial encystment are poorly studied. To uncover signal transduction components regulating encystment, ‘hyper-cyst’ mutants that form cysts under nutrient rich conditions were isolated through transposon mutagenesis. A subset of these mutations mapped to components of an operon (che3) of genes normally associated with bacterial chemotaxis. In normal chemotactic signaling, an inner membrane bound protein (Mcp) regulates the kinase activity of a cytostolic sensor kinase (CheA) through a coupling protein (CheW) in response to varying stimulus. CheA is a phospho-donor to CheY, which alters flagellar rotation via interactions with the flagellar complex. The system adapts to background levels of stimulus by altering CheA activity through reversible methylation of cytostolic Mcp glutamate residues by a methyl-transferase (CheR) and a methyl-esterase (CheB). The che3 operon contains all of these typical “che-like” genes including an additional sensor kinase-response regulator hybrid, cheS3. Deletions of these components are either early or delayed in encystment. Through in vitro kinase and phospho-transfer assays, as well as phenotypic analyses of chromosomal point mutants, we have begun to dissect how the che3 signaling pathway that controls differentiation, differs from that of the classical chemotaxis response.