| Summary: | Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Biology, 2004. === Includes bibliographical references. === Bacteria keep track of their population density by using extracellular signaling molecules. In response to the accumulation of these signals, changes in gene expression coordinate the switch from unicellular activity to group behavior. In Bacillus subtilis, this shift towards social behavior occurs, in part, through a signaling pathway that monitors the concentration of the ComX pheromone. ComX stimulates the receptor ComP which activates the transcription factor and response regulator ComA, resulting in the regulation of competence development and other processes. I characterized the signaling pathway and the ComA-mediated cellular response by examining global changes in gene transcription. Under the conditions tested, the ComX-P-A pathway is linear; the ComX pheromone acts exclusively on the ComP membrane histidine kinase receptor, which in turn only activates the response regulator ComA. I found that the activation of the ComX-P-A pathway leads to changes in expression of at least 35 transcriptional units; nine of them appear to be directly regulated by ComA. Many of the operons encode genes that affect the membrane and extracellular environment. The signaling pathway positively regulates transcription of genes involved in fatty acid metabolism, antibiotic production, exopolysaccharide biosynthesis, and degradative enzymes. The presence of signal sequences suggests that more than half of the ComA-dependent gene products of unknown function are membrane or extracellular in nature. These physiological changes may allow B. subtilis to become more resistant to the accumulation of toxic products, scavenge alternative nutrient sources, and eliminate possible competitors for those new nutrients. === (cont.) I also analyzed the srfA antibiotic operon, which is directly regulated by ComA. The srfA operon affects transcription of several indirect-ComA targets, including the Fap (fatty acid production) regulon and the antibiotic (skf) operon which regulates timing of sporulation. A product of the 5' end of the srfA operon affects the activity, rather than the stability, of the FapR repressor. A product from the 3' end of the operon may also be involved. In contrast, a product of the 3' end of the srfA operon helps regulate transcription of the skf operon. Irrespective of direct or indirect regulation, these physiological changes allow B. subtilis to adapt to a crowded environment where it competes for limited resources. Changing the membrane and extracellular environment under conditions of high cell density appears to be a conserved response in both Gram-positive and Gram-negative bacteria. === by Natalia Comella. === Ph.D.
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