A novel signal transduction system and feedback loop regulate fructan hydrolase gene expression in Streptococcus mutans.

The fruA gene of Streptococcus mutans encodes for a secreted fructan hydrolase (fructanase), an established virulence determinant required for releasing D-fructose from levan- and inulin-type fructans. Expression of fruA is under the control of carbon catabolite repression and is induced by growth in fructans. In this report, we identified an operon in S. mutans UA159 encoding a two-component system flanked by two predicted carbohydrate-binding proteins that is absolutely required for the expression of fruA. All four genes were found to be required for optimal growth of S. mutans on inulin-containing medium and for transcriptional activation of fruA. Complementation assays using a plasmid expressing the response regulator suggested that the two-component system works in concert with the sugar-binding proteins. This operon was also shown to activate a four-gene cluster located immediately downstream and encoding an Enzyme II (EII(Lev)) for a fructose/mannose sugar : phosphotransferase enzyme, which was found to negatively regulate the expression of fruA. Using transcriptional fusions, it was found that fructose could signal induction of the fruA and levD operons through the two-component system/sugar-binding protein complex. A recombinant LevR protein was shown to bind to the promoter regions of fruA and levD in gel mobility shift assays. Thus, a 'four-component signal transduction system' activates fructan catabolism and the expression of an Enzyme II complex that functions in a feedback loop to sense the accumulation of the end-product of fructan degradation.