Expression of gtfS is essential for normal insoluble glucan synthesis by Streptococcus downei.

The gtfI and gtfS genes of Streptococcus downei were investigated to determine the contribution of the respective enzymes to glucan production in the presence and absence of other glucosyltransferases. Extracts of Escherichia coli expressing cloned gtfS produced a short linear dextran from sucrose which could act as a primer for insoluble glucan synthesis when mixed with extracts of a strain expressing recombinant gtfI. To elucidate the contribution of gtfS to glucan production by S. downei, a mutant was constructed by insertionally inactivating gtfS. S. downei (gtfS mutant) colonies exhibited a marked phenotypic change on sucrose-containing media and a decreased ability to adhere to glass and produced no detectable water-insoluble glucan. These experiments confirm that expression of gtfS is essential for normal insoluble glucan synthesis by S. downei.