Characterization of Chemosensing in the Alphaproteobacterium <i> Azospirillum brasilense </i>

Motile bacteria must navigate their environment in constant search of nutrients to sustain life. Thus they have evolved precise and adaptable sensory systems to achieve this goal, making the navigation system of the model bacterium Escherichia coli the best characterized signal transduction pathway in Biology. However, many bacteria have evolved more sophisticated arsenals for sensing and responding to their environment including chemoreceptors to identify novel attractants in the microenvironment. The diazotrophic alphaproteobacterium Azospirillum brasilense inhabits the soil and colonizes the roots of cereals like rice, corn, and wheat. Like most proteobacterial, A. brasilense encodes multiple chemotaxis-like pathways, 4, of which only Che1 has been characterized in detail. Also, of the approximately 50 chemoreceptors encoded within the genome, only the function of AerC and Tlp1 have been determine and their role in energy taxis, the dominant behavior of A. brasilense. In this dissertation, I will describe the characterization of another chemoreceptor, Tlp2, with a sensing domain of unknown function and the role it plays in A. brasilense behavior. I will also describe my work in expanding knowledge of the chemotaxis-like pathway of Che1. Also, the role of Tlp1 in root colonization, chemotaxis, and aerotaxis, the ability to navigate oxygen gradients, has been published. My work will detail the role of the C-terminal PilZ domain, a domain shown to bind the ubiquitous bacterial second messenger cyclic-di-GMP. I will characterize the necessity of c-di-GMP binding to Tlp1 for cells to maintain the ability to remain sensitive to temporal changes in aeration. I will also discuss the novel role c-di-GMP plays in modulating the cell’s ability to remain motile and remain sensitive to addition changes in oxygen availability.