Characterization of the Function of the <i>Azospirillum brasilense</i> Che1 Chemotaxis Pathway in the Regulation of Chemotaxis, Cell Length and Clumping

Azospirillum brasilense is a gram-negative alphaproteobacterium that lives in the soil where it colonizes the root surfaces of cereals and grasses. The genome of A. brasilense has recently been sequenced and shown to possess four different chemotaxis-like operons. This dissertation project focused on characterizing the Che1 chemotaxis-like signal transduction pathway, which was initially implicated in regulation of the chemotaxis behavior. Deletions of individual genes within the Che1 pathway did not exhibit a null chemotaxis phenotype, leading us to investigate the role of this pathway in the lifestyle of A. brasilense and the mechanism(s) by which it functions. We have used a combination of microbiology and molecular genetics methods, including construction and characterization of several mutant strains lacking che1 genes, as well as molecular biology and microscopy. The data obtained suggest that Che1 is involved in regulating multiple cellular behaviors such as cell length and swimming speed, as well as having indirect effects on cell-to-cell clumping behavior, cell surface properties, and a minor role in regulating the motility bias. The data obtained also shed light on the function of the N-terminal HlyIII-like domain of CheA1 in A. brasilense which is also found in other bacterial species, as a single domain protein. Using A. brasilense, Escherichia coli and Bacillus subtilis, evidence is provided that HlyIII-like domains function to modulate membrane properties with effects on fatty acid composition that appear to also impair protein localization and function, including the control of cell length at division and chemotaxis.