[A further study of the nature of phenotypical reversions in thymidine phosphorylase deletion mutants of Escherichia coli K-12].

Thymidine dependent (thy) strains of Escherichia coli carrying deletions for thymidine phosphorylase (tpp gene) formed phenotypical reversions for the ability of growth at the medium containing thymine as a source of thymidilate. The ability of the thy tpp strains to utilyze thymine for growth is due to mutations of the regulatory genes cytR and udpR, which control the uridine phosphorylase activity. This enzyme in constitutive amounts catalyzes conversion of thymine to thymidine in bacterial cells with a block of deoxyribose catabolism (deoxyribomutase-negative strains). The udpR mutant by contrast to cytR mutants reported by Munch-Petersen et al. (1972) are shown to contain low, inducible levels of cytidine deaminase and deo-enzymes. The udpR mutation is recessive with respect to wild type allele. It is supposed that the product of udpR gene is a specific repressor of uridine phosphorylase (udp) gene. The udpR mutation is closely linked (90% contransduction) to uridine phosphorylase (udp) gene, and is located approximately at 75 min, on the E. coli chromosome.