Streptococcus downei

Abstract: Analysis of the derived amino acid sequences of toxins A and B from Clostridium difficile has identified an extraordinarily large number of repeat amino acid units in the C-terminal regions of the proteins. Nearly one third of each of the proteins consist of repeating units which appear, at least in the case of toxin A, to be responsible for carbohydrate binding. Similar repeat units are also found in the C-terminal region of four glucosyltransferases from Streptococcus mutans and Streptococcus downei, and in four lytic enzymes from Streptococcus pneumoniae and its bacteriophages (HB-3, Cp-1 and Cp-9). In each case the repeats constitute the ligand-binding portion of the respective enzymes. A glucan-binding protein from S. mutans, which lacks enzymatic activity, has similar repeats spanning almost the entire molecule. This family of ligand-binding proteins appears to be of modular design, with one module consisting of a repetitive ligand-binding domain located in the C-terminal region and the other module(s) providing enzymatic functions.

Abstract: The nucleotide sequence of the gbp gene, which encodes the glucan-binding protein (GBP) of Streptococcus mutans, was determined. The reading frame for gbp was 1,689 bases. A ribosome-binding site and putative promoter preceded the start codon, and potential stem-loop structures were identified downstream from the termination codon. The deduced amino acid sequence of the GBP revealed the presence of a signal peptide of 35 amino acids. The molecular weight of the processed protein was calculated to be 59,039. Two series of repeats spanned three-quarters of the carboxy-terminal end of the protein. The repeats were 32 to 34 and 17 to 20 amino acids in length and shared partial identity within each series. The repeats were found to be homologous to sequences hypothesized to be involved in glucan binding in the GTF-I of S. downei and to sequences within the protein products encoded by gtfB and gtfC of S. mutans. The repeated sequences may represent peptide segments that are important to glucan binding and may be distributed among GBPs from other bacterial inhabitants of plaque or the oral cavity.

Abstract: The complete nucleotide sequence was determined for the Streptococcus downei (previously Streptococcus sobrinus) MFe28 gtfS gene which specifies a glucosyltransferase (GTF-S) producing water-soluble glucan. A single open reading frame which encodes a mature protein with a molecular weight of 147,408 (1,328 amino acids) and a putative signal peptide 36 or 37 amino acids in length was detected. GTF-S shares extensive sequence similarity with GTF-I (gtfI) from S. downei and GTF-I (gtfB) and GTF-SI (gtfC) from Streptococcus mutans. GTF-S contains a highly conserved enzymatic domain and C-terminal repeated sequences which appear to be involved in glucan binding. Comparison of the deduced GTF-S protein sequence with other sequenced GTF genes of mutans streptococci revealed that these C-terminal repeats occurred in all cases, although the patterns of repeated sequences varied with respect to each other and to the glucan-binding protein of S. mutans. GTF-S contains four C-terminal repeat sequences ranging from 49 to 51 amino acids in length and a partial repeat of 13 amino acids. Nuclear magnetic resonance analysis of the glucan produced by GTF-S revealed that the product consisted of more than 90% alpha-1,6-linked glucosyl residues.

Abstract: A real-time polymerase chain reaction (PCR) assay was developed for the quantification of Streptococcus mutans. Primers targeting gtf genes of S. mutans were designed and tested for their specificity using 28 oral streptococcal strains, three other bacterial strains, and human DNA. The primers could amplify specifically the target DNA fragment from a mixture of oral streptococcus genomic DNA containing about 10 fg to 10 ng of S. mutans genome DNA. The real-time PCR produced a linear quantitative detection range over concentrations spanning seven exponential values, with a detection limit of a few copies of S. mutans’ genomic DNA per reaction tube. The results of the real-time PCR assay corresponded well to those of conventional culture assays for S. mutans in saliva samples. A real-time PCR assay for Streptococcus sobrinus and Streptococcus downei was also established and produced results that corresponded well to those from conventional culture assays for S. sobrinus in saliva samples. These assays will be useful as a new means to assess one of the important risk factors for caries.