Search

1. Valine at a concentration of as low as 0.1μg per ml inhibited growth of strain W3110-215 (λ-F-thy-deoC-), a derivative of Escherichia coli K12, after shift-down from nutrient broth to minimal medium.2. The inhibition by valine at this low concentration was overcome by 0.05 μg or more of isoleucine per ml.3. On agar medium, valine-inhibition was reversed with unique recovery patterns by alanine, homoserine, leucine, methionine and threonine, suggesting the differences in their mechanisms of action.4. Cells acquired resistance to 0.1μg of valine per ml on incubation for more than 40min in minimal medium containing threonine. The growth of these cells was still sensitive to 1.0μg or more of valine per ml. Cells incubated in the medium without threonine remained sensitive to 0.1μg of valine per ml throughout the incubation period of 6h.5. The cells acquired resistance because threonine enhanced derepression of synthesis of acetohydroxy acid synthase, which is sensitive to valine, and maintained the repressed condition of the valine-transport system of the cells.

The mechanism of the enhancing effect of nucleosides on catabolite repression of β-galactosidase synthesis by glucose or glycerol was studied. For this purpose, mutants of Escherichia coli strain 3000 deficient in the catabolic enzymes, thymidine phosphorylase, purine nucleoside phosphorylase, and phosphodeoxyribomutase were isolated. Strain W3110-215 (λ-, thy-, deoC-), a derivative of strain W3110 (λ-, prototroph), deficient in deoxyriboaldolase was also used.Catabolite repression of β-galactosidase synthesis by glucose was enhanced by adenosine but not by thymidine in a mutant deficient in thymidine phosphorylase, by thymidine but not adenosine in a mutant deficient in purine nucleoside phosphorylase, and by both thymidine and adenosine in a mutant deficient in phosphodeoxyribomutase. In strain W3110-215, adenosine increased catabolite repression of β-galactosidase by glycerol, whereas thymidine inhibited the synthesis completely after a lag of about 20 min in the presence or absence of glycerol, probably due to accumulation of some growth inhibitory substance, such as deoxyribose 5-phosphate (or its derivative(s)) formed from thymidine.These results suggest that nucleosides must be catabolized further than deoxyribose 1-phosphate or ribose 1-phosphate to exert an enhancing effect on repression of β-galactosidase by glucose or glycerol.

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.

After nutritional shift-down, threonine, α-ketobutyrate, or isoleucine with- in certain concentration range stimulated derepression of the synthesis of acetohydroxy acid synthase in strain W3110-215, a low-thymine requiring derivative of Escherichia coli K12. The acetohydroxy acid synthase induced by isoleucine was valine-sensitive. Since chloramphenicol inhibited the increase of the enzyme activity, the increase seemed to be due to enhancement of enzyme synthesis and not to the activation of the enzyme. From these results, it was suggested that not only limitation of leucine or valine, but also the presence of excess threonine or isoleucine is necessary for efficient derepression of the synthesis of acetohydroxy acid synthase.On the other hand, threonine did not stimulate the derepression of the synthesis of the enzyme in strains W2252 and 58-161, methionine-auxotrophic mutants of E. coli K12, growing in minimal medium containing methionine. When a prototrophic strain with regard to methionine requirement was used, methionine or threonine added in minimal medium derepressed synthesis of acetohydroxy acid synthase, but the simultaneous addition of the two compounds did not give any synergistic effect on the enzyme synthesis. The mechanisms of these phenomena are unknown.