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Characterization of the Streptococcus gordonii chromosomal region immediately downstream of the glucosyltransferase gene

Department of Oral Surgery and Hospital Dentistry, School of Dentistry^1,2 and Department of Microbiology and Immunology,¹ School of Medicine, Indiana University, Indianapolis, IN 46202, USA

Author for correspondence: M. M. Vickerman. Tel: +1 317 278 3250. Fax: +1 317 278 6244. e-mail: mvickerm{at}iupui.edu

The Streptococcus gordonii glucosyltransferase gene, gtfG, is positively regulated by the upstream determinant rgg. In the present study, two ORFs, transcribed on the opposite DNA strand, were identified immediately downstream of gtfG. The first, designated dsg, shares a convergent putative transcriptional terminator with gtfG, and encodes a predicted 46 kDa transmembrane protein similar to the Yersinia enterocolitica TrsA involved in polysaccharide biosynthesis. Insertional inactivation of dsg resulted in only 60% of the parental level of glucosyltransferase activity. The 870 bp gene 5' to dsg is similar to the gtfG regulatory determinant. Designated rggD, this rgg-like determinant downstream of gtfG encodes a putative 33·6 kDa cytoplasmic protein. Despite their sequence similarity, the functions of rgg and rggD appear specific. Strains in which rggD was insertionally inactivated and strains containing plasmid-borne rggD had parental levels of glucosyltransferase activity. Northern blot hybridization analyses showed 1·3 kb dsg-specific and 1·0 kb rggD-specific mRNA transcripts associated with this region; no polycistronic transcript was observed. Although rgg-like gene products have been demonstrated to function as positive transcriptional regulators of adjacent genes in several streptococcal species, Northern blot analysis suggested that rggD did not influence the transcription of dsg or the divergent downstream ylbN-like determinant under the conditions in the present study. Comparison of this S. gordonii chromosome region to other streptococcal genomes, which do not contain the rgg/rggD-flanked region involved in glucan synthesis, raised intriguing possibilities about the origins of this chromosomal region, and also suggested that rggD might regulate a distally located gene.

Keywords: oral streptococci, glucans, rgg

Abbreviations: GTF, glucosyltransferase; IPCR, inverse PCR; Spp, sucrose-promoted phenotype

The nucleotide sequence of the S. gordonii chromosomal region reported in this paper has been appended to the gtfG entry in GenBank with accession number U12643.

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