Transposon mutagenesis and marker rescue were used to isolate and identify an 8.5-kb contiguous region containing six open reading frames constituting the operon for the sorbitol P-enolpyruvate phosphotransferase transport system (PTS) of Streptococcus mutans LT11. The first gene, srlD, codes for sorbitol-6-phosphate dehydrogenase, followed downstream by srlR, coding for a transcriptional regulator; srlM, coding for a putative activator; and the srlA, srlE, and srlB genes, coding for the EIIC, EIIBC, and EIIA components of the sorbitol PTS, respectively. Among all sorbitol PTS operons characterized to date, the srlD gene is found after the genes coding for the EII components; thus, the location of the gene in S. mutans is unique. The SrlR protein is similar to several transcriptional regulators found in Bacillus spp. that contain PTS regulator domains (J. Stülke, M. Arnaud, G. Rapoport, and I. Martin-Verstraete, Mol. Microbiol. 28:865-874, 1998), and its gene overlaps the srlM gene by 1 bp. The arrangement of these two regulatory genes is unique, having not been reported for other bacteria.Oral streptococci, particularly aciduric species such as Streptococcus mutans, contribute to dental caries by degrading dietary sugars and sugar alcohols to metabolic acid end products, resulting in the demineralization of tooth mineral (4). Caries formation in the presence of readily fermentable carbohydrates, such as sucrose, has led to the use of low-cariogenic sugar substitutes, such as sorbitol (glucitol), in sugar-free gums and lozenges (3). More recently, however, the frequent use of sorbitol-containing products has been shown to result in increased levels of sorbitol-utilizing bacteria due to adaptation to sorbitol. The major sugar transport process in S. mutans is via the phosphoenolpyruvate: sugar phosphotransferase system (PTS) (17, 28), a group translocation process utilizing phosphoenolpyruvate as a substrate in phosphoryl transfer involving the general, non-sugar-specific proteins enzyme I and HPr and ultimately the sugar-specific, membrane-bound enzyme II (EII) complex, resulting in the transport and phosphorylation of the specific sugar being transported. The EII complexes are normally comprised of three functional domains, fused either within a single protein or on separate proteins, with domains IIA (formerly enzyme III) and IIB possessing the first and second phosphorylation sites, respectively, while the IIC domain forms the transmembrane channel and the sugar-binding site (17).Early work with S. mutans revealed that sorbitol transport by glucose-grown cells required the concomitant induction of the sorbitol-PTS and sorbitol-6-phosphate dehydrogenase (SDH), resulting in the formation of fructose-6-phosphate (8, 19). Sorbitol-PTS and SDH activities were repressed by low concentrations of glucose (8, 19) by a mechanism that was at least in part due to inducer exclusion, a mechanism not observed with glucose-PTS-negative mutants. Sorbitol transport by Streptococcus sanguis also occurs via an inducible sorbitol...
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