Streptococcus gordonii has a single glucosyltransferase (Gtf) enzyme that synthesizes glucans which can facilitate bacterial accumulation on hydroxyapatite surfaces. The Gtf structural gene, gtfG, is located immediately downstream from its positive transcriptional regulator, rgg. Regulation of Gtf activity appears to involve interaction of the rgg gene product with the DNA region flanking the gtfG promoter. We have previously identified a spontaneous mutant of S. gordonii in which a single point mutation near the 3-prime end of rgg resulted in a ca. 70% decrease in Gtf activity. Objective: It was hypothesized that this loss of activity was due to a change in the DNA binding site of trans-acting Rgg, or due to a change in the protein itself. Methods: To investigate these possibilities, genetic and biochemical analyses were performed. Histidine-tagged parental and mutant Rgg were expressed in E. coli and the properties of the purified proteins compared. Results: Chromosomal deletion of the DNA region containing the point mutation did not affect the ability of plasmid-borne rgg to increase Gtf activity. Plasmid-borne mutant rgg was unable to increase beta-galactosidase activity in gtfG promoter:lacZ fusions in E. coli to the extent of plasmids carrying parental rgg. Neither was plasmid-borne mutant rgg able to increase Gtf activity to the same extent as plasmids carrying the parental rgg in S. gordonii. Conclusion: The results suggest that the decreased Gtf activity seen in the S. gordonii strain with a mutation in the 3-prime end of rgg, was due to a change in activity of the encoded Rgg, rather than a change in an essential nucleotide in the DNA binding site. These studies provide insights into the molecular basis of S. gordonii Gtf regulation and specificity of Rgg-like proteins.

Back to the Microbiology / Immunology and Infection Control Program
Back to the IADR/AADR/CADR 80th General Session (March 6-9, 2002)

Top Level Search