Abstract
1. Biochemical characteristics of two types of thymine-less mutant of E. coli K12 with high and low thymine requirements (Thy20- and Thy2-) were compared.
2. The excretion of deoxyribose into the medium by Thy2- during thymine starvation was a unique character associated with the mutation of Thy20- to Thy2-, suggesting a deficiency of phosphodeoxyribomutase (Mut) and/or deoxyriboaldolase (Ald) whichparticipate in deoxyribonucleoside catabolism.
3. The mutation also results in change in the activities and inducibilities of thymidine phosphorylase (Tpp), purine nucleoside phosphorylase (Pup) and deoxyriboaldolase (Ald). Thy2- mutants were classified into four groups on the characters of these enzymes; 1) constitutive and may lack Mut, 2) inducible by thymidine or thymine starvation and lack Ald, 3a) non-inducible and may lack Mut, and 3b) non- or only slightly inducible and lack Ald.
4. Our data together with those of genetical studies show that a) the gene specifying all four enzymes participating in the catabolism of deoxyribonucleosides-Tpp, Pup, Ald, and Mut-might constitute an operon. The inducer for the operon is suggested to be dR-1-P. b) Accumulation of dR-5-P in the cells inhibits bacterial growth.
