Sinorhizobium meliloti are bacteria capable of metabolizing a broad range of carbon sources. The organism's ability to preferentially utilize the most preferred carbon source before metabolizing less energy efficient carbon compounds relies heavily on the process of catabolite repression. While succinate-mediated catabolite repression has been heavily studied and understood, glucose-mediated catabolite repression of lactose utilization has also been shown to occur within S. meliloti, yet the biological machinery necessary for this process to occur remains largely unknown. In order to discover the means by which glucose-mediated catabolite repression occurs in S. meliloti, twenty-two Tn5 mutants were assayed to determine if B-galactosidase was over-expressed in the presence of glucose. All mutants showed low levels of B-galactosidase when grown on glucose plus lactose, indicating that all were able to exert glucose-mediated catabolite repression on lactose utilization. However, the three mutants, 307620, 20749, and 207119, that were identified as glucose minus mutants prior to the assays also exhibited glucose-mediated catabolite repression of lactose metabolism. Mutant 107119, which behaved similarly to mutant 307620, was also observed to exhibit glucose-mediated catabolite repression of the lac operon. Since the three glucose minus mutants were unable to successfully utilize glucose, all four mutants were grown on a variety of carbon sources that are catabolized at different points within the Entner-Doudoroff pathway to determine which enzymes had been mutated respective to each mutant. By identifying which enzymes were defective in each glucose minus mutant, additional information was gained on how far glucose must be metabolized to exert glucose-mediated catabolite repression on lactose utilization. From this research, it appears that glucose must be processed as far as 6-phosphogluconate to exert glucose-mediated catabolite repression in S. meliloti.
DeNolf, R. (2009). Growing Glucose Minus Mutants on Entner-Doudoroff Pathway Carbon Sources to Explore Glucose-Mediated Catabolite Repression in Sinorhizobium meliloti (Undergraduate honors thesis, University of Redlands). Retrieved from http://inspire.redlands.edu/cas_honors/139