MOLECULAR ANALYSIS OF REPRESENTATIVE STREPTOCOCCUS-GORDONII SPP PHASEVARIANTS REVEALS NO DIFFERENCES IN THE GLUCOSYLTRANSFERASE STRUCTURALGENE, GTFG

Streptococcus gordonii glucosyltransferase polymerizes sucrose to form glucans, which confer a hard, sucrose-promoted phenotype (Spp(+)) to colonies on sucrose agar plates. The glucosyltransferase structural ge ne, gtfG, is positively regulated by the upstream determinant, rgg. St rain Challis undergoes a spontaneous, reversible phase variation betwe en high (Spp(+)) and low (Spp(-)) levels of glucosyltransferase activi ty. Representative strains were examined to gain insights into the bas is of glucosyltransferase phase variation. Western blots indicated tha t the level of glucosyltransferase activity was related to the amount of extracellular glucosyltransferase protein produced by Spp(-) and Sp p(+) strains. The nucleotide sequence of rgg and gtfG of the Spp(-) st rain CH97 was found to be identical to that of the Spp(+) parent, indi cating that DNA differences in these regions are not the basis for glu cosyltransferase phase variation. Indeed, C-13-NMR spectroscopy sugges ted that glucans synthesized by strain CH97 glucosyltransferase were s imilar to those synthesized by glucosyltransferase of the Spp(+) paren tal strain, indicating a quantitative rather than qualitative change. However, one Spp(-) strain, CH1C1, had a point mutation in rgg; replac ement of the parent rgg with the CH1C1 allele resulted in decreased le vels of glucosyltransferase protein and activity. The results indicate that glucosyltransferase phase variation can occur in more than one w ay, and suggest that glucosyltransferase regulation may involve distal ly located regulatory gene(s) that affect rgg and/or gtfG expression.