Regulation of purine biosynthetic genes expression inSalmonella typhimurium IV Oc mutation site ofpurG and its function analysis

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 mug/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.

Study of adaptive mutations in Salmonella typhimurium by using a super-repressing mutant of a trans regulatory gene purR

Salmonella typhimurium purR encodes a transcriptional repressor regulating gene expression of de novo purine nucleotide biosynthesis. It represses purD gene transcription by binding to the 16-base pair purD operator (PUR box). A S. typhimurium strain carrying a super-repressing mutant of purR, purR(s), has been used as an experimental system to study adaptive mutation. Escherichia coli lac genes were genetically engineered into S. typhimurium chromosome and repressed by purR(s) so that they could be used as an indicator of adaptive mutations in purR(s) or in the purD operator. Mutations in purR(s) or in the purD operator accumulated when the mutant strain was placed on a minimal lactose plate supplemented with 10 microg/ml of adenine during prolonged incubation. These specific mutations reverted the mutant strain from lac(-) to lac(+) phenotype. The lac(+) strains were categorized into the early- and late-arising mutants according to the time for colony appearance. Our genetic studies indicate that (i) Poisson distributed mutations accumulated in the chromosomal regulatory gene purR or the purD operator in very slowly dividing cells under selection; (ii) after about 8 days of selection, the frequency of mutations in purD operator reached the high value of about two mutations per 10(8) cells; (iii) the mutational spectrum in the purD operator during growth was not significantly different from that during selection; (iv) defects in mutL or mutS appeared to have a stronger effect on growth-dependent mutations than on adaptive mutations.