Gage Lab-Research

Some of the genes required for S. meliloti to nodulate its host plant have been the targets of an extensive amount of research. Of growing interest to researchers are bacterial activities and processes which are not uniquely involved in differentiation, but are nevertheless important for symbiosis. S. meliloti can utilize a large number of compounds as sources of carbon for growth. Dicarboxylic acids such as succinate, fumarate and malate are of special importance for S. meliloti: oxidation of such compounds is essential for nitrogen fixation, they support a high growth rates and they are favored carbon and energy sources. S. meliloti uses them in preference to many other carbon sources. The phenomenon of using succinate in preference to less favored carbon sources is referred to here as “succinate-mediated catabolite repression”.

The mechanisms of catabolite repression are well documented in E. coli, S. typhimurium and B. subtilis. In these, glucose is a repressing sugar, and catabolite repression is mediated by passage of glucose through the PTS transport system. Passage of glucose through the PTS ultimately causes secondary carbon sources to be excluded from the cell. In addition, in E. coli and S. typhimurium, glucose transport through the PTS results in low cAMP levels which prevents transcription of many genes and operons needed for catabolism of secondary carbon sources.

While the phenomenon of succinate-mediated catabolite repression in S. meliloti is well documented, the molecular mechanisms of its operation remain to be elucidated. It is clear that succinate-mediated catabolite repression in S. meliloti must be different than catabolite repression in E. coli, S. typhimurium and B. subtilis. This is because the preferred carbon sources in S. meliloti do not enter through the PTS (in fact S. meliloti appears not have a complete PTS), but rather through a very different transporter, the DctA permease.

We have been identified and characterized genes required for galactoside utilization in S. meliloti. These genes are not expressed when galactosides are present along with succinate; that is they show catabolite repression in response to succinate. We also have shown using genetics and biochemistry that the accumulation of galactosides in S. meliloti is prevented when succinate is present.

Recently we have identified, through genetic methods, genes required to bring about succinate-mediated catabolite repression. These genes arecentral to catabolite control and some will likely directly mediate the events which prevent the accumulation of secondary carbon sources. Also, the genes clearly participate in signaling events that connect the presence of succinate to its effects on carbon utilization and gene expression.

Because the repressing carbon source, succinate, is not transported by the PTS system, the mechanisms of catabolite repression in S. meliloti should be quite interesting, and substantially different from those described in other bacteria. In addition to understanding how this genome-wide control system operates in S. meliloti our work should also lead to insights about genome-wide control in other organisms that use succinate, and related compounds, as preferred carbon sources. Some of these other organisms include pathogens and industrially and ecologically important species, from the Pseudomonad family.




		Gene expression in S. meliloti
	Some of the genes required for S. meliloti to nodulate its host plant have been the targets of an extensive amount of research. Of growing interest to researchers are bacterial activities and processes which are not uniquely involved in differentiation, but are nevertheless important for symbiosis. S. meliloti can utilize a large number of compounds as sources of carbon for growth. Dicarboxylic acids such as succinate, fumarate and malate are of special importance for S. meliloti: oxidation of such compounds is essential for nitrogen fixation, they support a high growth rates and they are favored carbon and energy sources. S. meliloti uses them in preference to many other carbon sources. The phenomenon of using succinate in preference to less favored carbon sources is referred to here as “succinate-mediated catabolite repression”. The mechanisms of catabolite repression are well documented in E. coli, S. typhimurium and B. subtilis. In these, glucose is a repressing sugar, and catabolite repression is mediated by passage of glucose through the PTS transport system. Passage of glucose through the PTS ultimately causes secondary carbon sources to be excluded from the cell. In addition, in E. coli and S. typhimurium, glucose transport through the PTS results in low cAMP levels which prevents transcription of many genes and operons needed for catabolism of secondary carbon sources. While the phenomenon of succinate-mediated catabolite repression in S. meliloti is well documented, the molecular mechanisms of its operation remain to be elucidated. It is clear that succinate-mediated catabolite repression in S. meliloti must be different than catabolite repression in E. coli, S. typhimurium and B. subtilis. This is because the preferred carbon sources in S. meliloti do not enter through the PTS (in fact S. meliloti appears not have a complete PTS), but rather through a very different transporter, the DctA permease. We have been identified and characterized genes required for galactoside utilization in S. meliloti. These genes are not expressed when galactosides are present along with succinate; that is they show catabolite repression in response to succinate. We also have shown using genetics and biochemistry that the accumulation of galactosides in S. meliloti is prevented when succinate is present. Recently we have identified, through genetic methods, genes required to bring about succinate-mediated catabolite repression. These genes arecentral to catabolite control and some will likely directly mediate the events which prevent the accumulation of secondary carbon sources. Also, the genes clearly participate in signaling events that connect the presence of succinate to its effects on carbon utilization and gene expression. Because the repressing carbon source, succinate, is not transported by the PTS system, the mechanisms of catabolite repression in S. meliloti should be quite interesting, and substantially different from those described in other bacteria. In addition to understanding how this genome-wide control system operates in S. meliloti our work should also lead to insights about genome-wide control in other organisms that use succinate, and related compounds, as preferred carbon sources. Some of these other organisms include pathogens and industrially and ecologically important species, from the Pseudomonad family.