Catabolism of nucleosides is essential for their effects in enhancing catabolite repression of .BETA.-galactosidase synthesis in Escherichia coli K12.

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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.
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