[Phenotypic manifestation of the pnd mutation, which promotes purine nucleoside cleavage by Escherichia coli K-12 cells, in the genome of strains defective in the metabolism of nucleic acid precursors].

Strains of Escherichia coli K-12 containing both pnd1 mutation, rendering bacteria capable to catabolize purine nucleosides without participation of purine nucleoside phosphorylase (pup gene), and mutations in several genes of purine metabolism or nucleosides catabolism have been constructed. The introduction of the deletion mutation in adenosine deaminase gene (add) into the pup pnd genome does not affect the ability of mutants to utilize adenosine and deoxyadenosine as the sole carbon and energy sources. Mutations affecting purine phosphoribosyltransferases (hpt and gpt) block the ability of pup pnd mutants to utilize hypoxanthine, guanine and their deoxyribonucleosides and also xanthine and xanthosine as the only purine source. A mutation in deoxyribomutase (drm) disturbs the ability of pnd mutants to use all purine ribo- and deoxy-ribonucleosides as carbon and energy sources, whereas a mutation in deoxyriboaldolase (dra) only disturbs utilization of deoxyribonucleosides. These data seem to indicate that the activity promoted by pnd mutations catalyzes the cell reaction of irreversible phosphorolytic cleavage of the N-glycoside bond of the purine nucleosides molecules: purine nucleoside + phosphate leads to purine + pentose-1-phosphate. It is suggested that pnd mutations affect the structural gene of some phosphorolytic enzyme and modify its substrate specificity. Evidence is presented that the structural gene of a new nucleoside phosphorylase is not sensitive to catabolite repression.