Regulated expression of the Streptococcus mutans dlt genes correlates withintracellular polysaccharide accumulation

Intracellular polysaccharides (IPS) are glycogen-like storage polymers whic h contribute significantly to Streptococcus mutans-induced cariogenesis. We previously identified and cloned a locus from the S. mutans chromosome whi ch is required for the accumulation of IFS. Sequencing of this locus reveal ed at least four contiguous open reading frames, all of which are preceded by a common promoter region and are transcribed in the same direction. Anal ysis of the amino acid sequence deduced from the first of these open readin g frames (ORF1) revealed domains which are highly conserved among D alanine -activating enzymes (DltA) in Lactobacillus rhamnosus (formerly Lactobacill us casei) and Bacillus subtilis. The deduced amino acid sequences derived f rom ORF2, -3, and -4 also exhibit extensive similarity to DltB, -C, and -D, respectively, in these microorganisms. However, Southern hybridization exp eriments indicate that this operon maps to a locus on the S. mutans chromos ome which is separate from the glgP, glgA, and glgD genes, whose products a re known mediators of bacterial IFS accumulation. We therefore assigned a n ew dlt designation to the locus which we had formerly called gig, We mainta in that the dlt genes are involved in S. mutans IFS accumulation, however, since they complement a mutation in trans which otherwise renders S. mutans IFS deficient, In this study, we found that expression of the S. mutans dl t genes is growth phase dependent and is modulated by carbohydrates interna lized via the phosphoenolpyruvate phosphotransferase system (PTS). We demon strated that the S. mutans dlt genes are expressed constitutively when non- PTS sugars are provided as the sole source of carbohydrate. Consistent with a role for the PTS in dlt expression is a similar constitutive expression of the nit genes in an S, mutans PTS mutant grown in a chemically defined m edium supplemented with glucose. In summary, these findings support a novel role for the dlt gene products in S. mutans IFS accumulation and suggest t hat dlt expression in this oral pathogen is subject to complex mechanisms o f control imposed by growth phase, dietary carbohydrate, and other factors present in the plaque environment.