Characterization of the product of the gtfS gene of Streptococcus downei, a primer-independent enzyme synthesizing oligo-isomaltosaccharides.

Summary: The gefS gene, coding for a glucosyltransferase which synthesizes water-soluble glucan and previously cloned from Streptococcus downei strain MFe28 (mutans serotype h) into a bacteriophage vector, was subcloned into a plasmid vector. The gtfS gene products expressed in Escherichia coli were compared to the primer-independent, oligoisomaltosaccharide synthase in Streptococcus sobrinus strain AHT (mutans serotype g) and shown to resemble it closely in molecular mass, isoelectric point, immunological properties, optimum pH and K m values. The glucans produced from sucrose by the gtfS gene products are α-1,6-linked linear oligo-isomaltosaccharides without any branching sites. A similar gtfS gene was also detected on chromosomal DNA from S. sobrinus strain AHT.