Streptococcus gordonii exhibits reversible phase variation between high (Spp+) and low (Spp-) levels of glucosyltransferase (GTF) activity. The molecular basis for this phase variation is unknown, but may involve frameshifts or rearrangements of DNA in a regulatory region. The gtfR region, immediately upstream from the gtfG structural gene has been implicated in regulating the level of gtfG expression. The proposed studies will use molecular genetic techniques to examine the expression of gtfR and gtfR in a Rec- Enterococcus faecalis background. Gene expression will be monitored using lacZ transcriptional fusions. DNA regions involved in the regulation of lacZ expression will be characterized, sequenced and compared to comparable regions in S. gordonii Spp+ and Spp- strains to attempt to determine the molecular basis for phase variation in these organisms. If additional chromosomal regions of S. gordonii are found to be involved in gtfR and/or gtfG expression, clones of these regions will be characterized. Appropriate constructs will be transformed back into S. gordonii strains to determine the effects that various culture conditions, known to produce qualitative and quantitative changes in GTF activity, may have upon gtfR and gtfG expression. The GTF enzyme and its glucan products have been implicated in mediating oral bacterial interactions. In vitro studies have shown that Spp+ and Spp- strains of the early plaque organism S. gordonii, display different affinities for various oral surfaces under different environmental conditions and use different mechanisms to colonize these surfaces, suggesting that in vivo GTF phase variation may allow differential colonization of oral sites. It is hoped that an understanding of the genes involved in the regulation of GTF activity will provide insight into oral microbial ecology and dental health.
