MOLECULAR AND GENETIC-ANALYSIS OF MULTIPLE CHANGES IN THE LEVELS OF PRODUCTION OF VIRULENCE FACTORS IN A SUBCULTURED VARIANT OF STREPTOCOCCUS-MUTANS

We previously isolated a variant strain, Xc100L, which shows decreased production of a surface protein antigen with a molecular mass of 190 kDa (PAc), after repeated subculturing of Streptococcus mutans strain Xe [Koga, T. et al. (1989) J. Gen. Microbiol. 135, 3199-3202]. In the present study, the levels of expression of the gtfB, gtfC, gtfD and ft f genes coding for polysaccharide-synthesizing enzymes in strain Xc100 L were compared with those in strain Xc. Western blot analysis reveale d multiple differences in the levels of production of these enzymes be tween these two strains. The amounts of the gtfB and gtfC gene product s responsible for water-insoluble glucan synthesis in strain Xc100L we re lower than those in strain Xc, whereas the amounts of the gtfD and ftf gene products responsible for water-soluble glucan synthesis and f ructan synthesis, respectively, in strain Xc100L were higher than thos e in strain Xc. Northern blot analysis revealed that the amounts of th e four enzymes and PAc produced by strain Xc100L reflected the relativ e amounts of mRNAs from the genes. The chloramphenicol acetyltransfera se gene was fused with each of these five genes, and the transcription al activity of each gene in strain Xc100L was quantitatively compared with that in strain Xc. The chloramphenicol acetyltransferase assay al so indicated that the phenotypic differences between strain Xc and str ain Xc100L were due to differences in the transcriptional activities o f the virulence genes. No differences in the nucleotide sequences of t he promoter regions of the gtfB, gtfC, gtfD, ftf and pac genes were fo und between strain Xc and strain Xc100L. It is possible that a factor( s) affecting the levels of transcription of the multiple virulence gen es exists in S. mutans.