It has been known that 10 enzymatic reactions are involved in de novo biosynthesis of IMP in Salmonella typhimurium. 10 genes encoding the enzymes have been localized on the Salmonella typhimurium chromosome. Seven structure genes including pur JHD operon are negatively regulated by purR. In this study the purJHD operon containing O+ and Oc were cloned in vivo respectively. Two preliminary clones, p2-9 (O+) and pC-12 (Oc) were identified to be the hybrid plasmids carrying intact pur JHD operon by genetic complementary test and reconstruction enzyme analysis. The fragments of O+ and Oc were subcloned on to pUC19 and DNA sequence were determined directly by DNA sequencer. The DNA sequence of O+ fragment revealed that a 16 base consensus sequence (named PUR box) GCGCAAACGTTTTCGT also existed in control region of pur JHD operon in Salmonella typhimurium. The DNA sequence of Oc fragment indicated that only one base change occurred at the 14th position of PUR box(C-->T). Our result strongly support the idea that PUR box is binding region of purR protein.