Abstract
Studies were made on mutants from a deoxyriboside-sensitive Thy- strain selected for their ability to grow on thymidine at high concentration, which still require a low concentration of thymine for growth. They were found to be divided into two groups with regard to the inducibilities of their thymidine phosphorylase and purine nucleoside phosphorylase. In one group these enzymes were inducible (inducible type) and in the other group they were not (non-inducible type). All the mutants showed lower thymidine phosphorylase activity than the parent strain. However, there were striking quantitative differences between non-inducible type strains, in the level of activity of purine nucleoside phosphorylase. The only exception was strain T429 which showed such high purine nucleoside phosphorylase activity that the enzyme seemed to be constitutive. All these thymidine-resistant mutants lacked deoxyriboaldolase. Some non- inducible type mutants also contained no phosphodeoxyribomutase, while others, as well as inducible type mutants contained this enzyme.
The gene(s) involved in the mutation causing these changes was mapped preliminary by transduction with phage Plc on a site which was closely linked to thr at the opposite side to leu where the deo-operon is present. This gene was concluded to be in the drm gene in some of the non-inducible type mutants, and not to be in the drm gene in the other non-inducible type mutants and in all inducible type thymidine-resistant utants.
