IDENTIFICATION OF A 4TH GENE INVOLVED IN DTDP-RHAMNOSE SYNTHESIS IN STREPTOCOCCUS-MUTANS

had isolated three genes (rmlA, rmlB, and rmlC) involved in dTDP-rhamn ose synthesis in Streptococcus mutans and found that three genes were insufficient for dTDP-rhamnose synthesis (Y.Tsukioka, Y. Yamashita, T. Oho, Y. Nakano, and T. Koga, J. Bacteriol. 179:1126-1134, 1997). The rmlD gene of S. mutans, encoding the enzyme which catalyzes the last s tep of dTDP-rhamnose synthesis, has been cloned and sequenced. The cel l extract of Escherichia coli expressing the rmlD gene of S. mutans ex hibited enzymatic activity corresponding to its counterpart in Shigell a flexneri, a gram-negative bacterium. Rhamnose aas not detected in th e cell wall preparation purified from the mutant in which the cloned g ene was insertionally inactivated. Rabbit antiserum against S. mutans serotype c-specific antigen did not react with autoclaved extracts fro m the mutant. The rmlD gene product of S. mutans compensated for the i ncompleteness of dTDP-rhamnose synthesis by the three previously isola ted genes. These results indicate that the rmlD gene product is indisp ensable for the dTDP-rhamnose pathway and subsequently for the synthes is of serotype-specific antigen in S. mutans. Furthermore, conservatio n of the rmlD gene in Streptococcus species was demonstrated by Southe rn blot analysis.