研究适应突变的一个新的实验系统

以鼠伤寒沙门菌嘌呤生物合成途径中一个反式调节基因purR的超阻遏突变体(purRS)为材料,利用该突变体在补加限量腺嘌呤和以乳糖为惟一碳源的NCE培养基上生长非常缓慢(不能形成菌落)的特性,观察到了编码阻遏蛋白的调节基因purR可在上述非致死选择条件下产生迟后突变现象.迟后突变体重建实验证明,此类突变体为非缓慢生长突变体;统计分析显示,迟后突变体呈Poisson分布,表明突变发生在选择平板上;共转导分析初步证明,迟后突变发生在被选择基因purR或purD 结构基因的cis元件16 bp PUR box.以上结果暗示,利用此实验系统观察到的迟后突变现象是与随机突变不同的适应突变的结果.将适应突变基因从碳代谢和氨基酸合成的结构基因扩大到编码阻遏蛋白的调节基因,既为适应突变的普遍性提供了新的证据,也为适应突变研究提供了一个新的实验系统.     

A new experimental system for study on adaptive mutations

A super-repressed mutant of purR (purRs), which encodes a repressor protein controlling expression of purine biosynthetic genes in Salmonella typhimurium, grew very slowly on NCE medium with 10 μg/mL Ade and lactose as sole carbon source (cannot form colonies). However, a phenomenon of late-arising mutations was observed when purRs mutants were spread on NCE+lactose plates and subjected to a prolonged non-lethal selection. The reconstruction experiments of revertants showed that the late-arising "lac+" mutants are not slow growing mutants. Statistical analysis indicated that the distribution of late-arising mutants is Poisson distribution, showing that reversion occurred after plating. The result of co-transductional analysis preliminarily showed that late-arising mutation occurred at selected gene purR or 16 bp PUR box, cis element of structural gene purD. The above results suggest that the phenomenon of late-arising mutation observed by our system is a result of adaptive mutations which are different     

A new experimental system for study on adaptive mutations

A super-repressed mutant of purR (purR^S), which encodes a repressor protein controlling expression of purine biosynthetic genes inSalmonella typhimurium, grew very slowly on NCE medium with 10 μg/mL Ade and lactose as sole carbon source (cannot form colonies). However, a phenomenon of late-arising mutations was observed when purR^S mutants were spread on NCE+lactose plates and subjected to a prolonged non-lethal selection. The reconstruction experiments of revertants showed that the late-arising “lac⁺” mutants are not slow growing mutants. Statistical analysis indicated that the distribution of late-arising mutants is Poisson distribution, showing that reversion occurred after plating. The result of co-transductional analysis preliminarily showed that late-arising mutation occurred at selected genepurR or 16 bp PUR box,cis element of structural genepurD. The above results suggest that the phenomenon of late-arising mutation observed by our system is a result of adaptive mutations which are different from random mutations. This is the first time to extend target genes at which adaptive mutations could occur from structural genes involved in carbon metabolism and amino acid biosynthesis totrans regulatory gene coding repressor protein. Our results have provided not only a new proof for generality of adaptive mutations but also a new system for study on adaptive mutations.