Salmonella typhimurium 5 phosphoribosylformylglycinamide (FGAR) amidotransferase encoded bypurG gene catalyzes the conversion of FGAR to formylglycinamide ribonucleotide (FGAM) in the presence of glu- tamine and ATP for thede novo purine nucleotide biosynthesis.purG gene is negatively regulated by a repressor-operator system. The O(+) purG and O(c) purG were cloned respectivelyin vivo. Restriction enzymes analysis of preliminary clones pLBG-1 (O(+)) and pLBG-2 (O(c)) were carried out. The hybrid plasmids pLB1933 (O(+)) and pLB1927 (O(c)) containing 5' control region ofpurG were constructed and the DNA sequences were determined respectively, DNA sequences data showed that O(c) mutation ofpurG occurred at the 3rd position of 16 bp PUR box in the 5' control region (G-->A). Gel retardation experiment indicated that the repressor bound well with O(+) PUR box, but not with O(c) PUR box. The result strongly supported the idea that PUR box is the binding region of repressor protein and the 3rd position base G of PUR box is essential for the binding function with repressor protein.