Tight Genetic Linkage of a Glucosyltransferase and Dextranase of Streptococcus mutans GS-5

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Abstract
A genetic library consisting of over 5000 clones with an average insert size of 6.9 kilobasepairs (kbp) of Streptococcus mutans GS-5 has been constructed in a bivalent plasmid vector pMK3, which is capable of replicating in Escherichia coli and Bacillus subtilis. The recombinant plasmid pSUCRI, containing a 6.0 kbp fragment of S. mutans GS-5 DNA, was the focus of this study. Using Southern hybridization, in vitro and in vivo gene expression techniques, and biochemical analysis, this clone was shown to encode the 55 kiloDalton (kDal) GS-5 gtfA gene product, as well as a 38 and a 66 kDal polypeptide. In addition to the gtfA gene, pSUCRI encodes a dextranase activity with specificity for α(1→6)-linked glucans, and with no detectable activity on mutan. The dextranase enzyme had an apparent molecular weight of 66 kDal as demonstrated by SDS-PAGE analysis of the proteins produced by a dextranase-negative deletion derivative. The pH optimum of the enzyme was approximately 6.0, and there was no detectable activity below pH 5.0. By subcloning various combinations of DNA fragments from pSUCRI, it was demonstrated that the dextranase gene (designated dexB) can be separated from the gtfA gene and still be efficiently expressed in both E. coli and B. subtilis. The dexB gene contained its own promoter and ribosome-binding site. The genetic linkage of the gtfA and dexB genes in the S. mutans GS-5 chromosome was confirmed by Southern hybridization and by the independent isolation of four distinct clones containing the gtfA gene and common flanking sequences. In addition to a glucosyltransferase and dextranase, an invertase-like activity is also encoded on pSUCRI, indicating that there is a cluster of genes on the S. mutans GS-5 chromosome which is devoted to the dissimilation of sucrose and concomitant synthesis or modification of glucans into a water-insoluble form, perhaps constituting an operon for glucan modification which can be coordinately regulated in response to environmental alterations.