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CtrA and GtaR : two systems that regulate the Gene Transfer Agent in Rhodobacter capsulatus Leung, Molly Mo-Yin

Abstract

Bacteria are found in almost all conceivable environments, and some species can survive many different conditions. The ability to detect environmental conditions and respond with appropriate changes to gene expression is essential to survival. Bacteria sometimes express genes involved in horizontal gene transfer when encountering a stressful environment. Horizontal gene transfer has an important role in the evolution of prokaryotic genomes. Rhodobacter capsulatus produces a mediator of horizontal gene transfer called the gene transfer agent (GTA). The R. capsulatus GTA is a bacteriophage-like particle that transfers ~4 kb of double stranded genomic DNA using a transduction-like mechanism. Previously, two proteins encoded outside the GTA gene cluster, GtaI and CtrA, were found to regulate GTA expression. GtaI and GtaR are LuxI-type and LuxR-type quorum sensing proteins, respectively. CtrA and CckA are homologues of the response regulator and sensor kinase, respectively, of the Caulobacter crescentus CtrA/CckA signal transduction system. In this thesis, I studied the interactions between these regulatory proteins, environmental conditions and GTA in R. capsulatus. I found that growth conditions had opposite effects on GTA and ctrA expression, but no effect on gtaR expression, and phosphate limitation decreased expression of ctrA. Knockout experiments revealed that GtaI and GtaR affect ctrA, gtaR and GTA expression. Results from GtaR-DNA binding experiments were consistent with a model in which GtaR directly regulates its own expression but indirectly regulates ctrA and GTA expression. These studies also identified GtaR binding sequences. I found that in R. capsulatus CtrA did not regulate its own transcription, contrary to what occurs in C. crescentus. My research also showed that GTA expression was affected by at least one other unidentified system. Promoter deletion studies of ctrA, gtaR and GTA genes identified sequences that may be involved in GtaI-, GtaR-, CtrA-, and/or growth condition-based regulation. Overall, these studies contribute to the understanding of how bacteria detect multiple environmental signals and respond with changes to gene expression.

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