Nucleotide sequence analysis of the gtfT gene from Streptococcus sobrinus OMZ176

The gtfT gene and its upstream region isolated from the Streptococcus sobrinus OMZ176 chromosomal DNA were sequenced. The gtfT gene was preceded by a potential Shine-Dalgarno sequence. The gtfT gene product, glucosyltransferase (GTF), displays a typical gram-positive bacterial signal peptide sequence and both an active site peptide sequence and carboxy-terminal repeats typical of GTFs. The signal sequence is similar to those of other known GTF proteins. The putative active-site peptide sequence of this enzyme was DGIRVDAVD, which was different by one amino acid from the active-site peptide sequence derived from two different types of the S. sobrinus GTFs reported previously (G. Mooser, S. A. Hefta, R. J. Paxton, J. E. Shively, and T. D. Lee, J. Biol. Chem. 266:8916-8922, 1991). The gtfT gene product has three repeated sequences of 51 to 52 amino acids and a partial repeat of 18 amino acids. Another open reading frame (ORF) was detected in the region immediately upstream of the gtfT gene. The upstream ORF showed substantial DNA homology with the gtfS gene isolated from Streptococcus downei MFe28. The inferred amino acid sequence of the upstream ORF has four repeating units and has extensive homology with the repeated peptides coded by the S. downei gtfS gene. These results suggested that the gtfT gene was a typical gtf gene isolated from the mutans streptococci and that the two gtf genes were located in tandem on the chromosomal DNA of S. sobrinus OMZ176.