Genetic analysis of thymidine resistant and low thymine requiring mutants of Escherichia coli K 12 induced by bacteriophage Mu 1

Abstract

Four genes, dra, tpp, drm, and pup, that specify enzymes involved in the catabolism of nucleosides and deoxynucleosides in E. coli are known to be very closely linked in the order dra tpp drm pup. By infecting cells with the phage Mu1 and isolating low thymine requiring derivatives of a strain lacking thymidylate synthetase and also thymidine resistant mutants of a dra- strain, it has been possible to select for strains in which Mu1 is inserted in this gene cluster. Making use of the polar effect of Mu induced mutations on more distal genes in the same transcriptional unit, evidence is presented that dra and tpp are cotranscribed from a promoter to the left of dra, and drm and pup are cotranscribed from a promotor located between tpp and drm. Residual levels of purine nucleoside phosphorylase in drm- mutants induced by phage Mu seem to indicate that a weak promotor lies between drm and pup. From a strain in which Mu1 is inserted in drm, a mutant has been isolated that has a deletion extending into tpp. Since this strain lacks thymidylate synthetase, it is unable to grow on minimal medium containing thymine. Mutants isolated from this strain that can grow on minimal medium containing thymine have been shown to have increased levels of the enzyme uridine phosphorylase.