Recombination between gtfB and gtfC Is Required for Survival of a dTDP-Rhamnose Synthesis-Deficient Mutant of Streptococcus mutans in the Presence of Sucrose

The rml genes are involved in dTDP-rhamnose synthesis in Streptococcus mutans. A gene fusion between gtfB and gtfC, which both encode extracellular water-insoluble glucan-synthesizing enzymes, accompanied by inactivation of the rml genes was observed for cells grown in the presence of sucrose. The survival rates of rml mutants isolated in the absence of sucrose were drastically reduced in the presence of sucrose. The rates were consistent with the frequency of spontaneous gene fusions between gtfB and gtfC, suggesting that the spontaneous recombinant organisms were selected in the presence of sucrose. The rml mutants with a gtfB-gtfC fusion gene had markedly reduced water-insoluble glucan synthetic activity and lost the ability to colonize glass surfaces in the presence of sucrose. These results suggest that the rml mutants of S. mutans, which are defective in dTDP-rhamnose synthesis, can survive only in the absence of water-insoluble glucan synthesis.

rml基因（rml genes）参与变形链球菌（Streptococcus mutans）中的dTDP-鼠李糖（dTDP-rhamnose）合成过程。在蔗糖存在条件下培养的细胞中，研究人员观察到gtfB与gtfC基因之间发生基因融合（gene fusion）——二者均编码胞外不溶性葡聚糖合成酶——同时伴随rml基因的失活。在无蔗糖条件下分离得到的rml突变体，其存活率在蔗糖存在时大幅降低。该存活率变化与gtfB和gtfC之间自发基因融合（spontaneous gene fusions）的频率相符，提示蔗糖存在条件下会筛选出自发重组体（spontaneous recombinant organisms）。携带gtfB-gtfC融合基因的rml突变体，其不溶性葡聚糖合成活性显著下降，且在蔗糖存在时丧失了在玻璃表面定殖的能力。上述结果表明，dTDP-鼠李糖合成存在缺陷的变形链球菌rml突变体，仅能在不进行不溶性葡聚糖合成的条件下存活。