[Escherichia coli K-12 mutants for the thymidine phosphorylase structural gene that retain the anabolic function of the enzyme].

From the Escherichia coli thymine auxotroph carrying a constitutive mutation for deo-enzymes (thy deoR) mutants (tpp38, tpp39 and tpp40) for thymidine phosphorylase (catalyzing a conversion of thymine to thymidine) were isolated via selection for a low thymine requirement. In the thy deoR+ genome these mutations led to inability of bacteria to use thymidine as the sole carbon source for growth, though the ability to utilize thymine was retained at the level comparable to that of thy deoR+ tpp+ bacteria. In the thy deoR genome mutations obtained led to a more efficient utilization of thymine in comparison with the thy deoR tpp++ strain. At the same time, the thymidine phosphorylase activity, as determined by a degradation of thymidine in bacterial extracts was lower in thy deoR strains carrying tpp38, tpp39 and tpp40 by factors 5,25 and 22, respectively, in comparison with the thy deoR tpp+ strain. The mutations tpp38, 39, 40 were localized in the distal part of the tpp structural gene (tpp39 and tpp40 being in the extreme distal position), whereas the earlier described tpp-leaky mutants incapable of using exogenous thymine for growth were mapped in the extreme proximal part of the tpp gene (Sukhodolets et al., 1971). It is proposed that the tpp-leaky mutations obtained enhance the thymidine phosphorylase affinity to deoxyribose-1-phosphate, a product in a reversible reaction of the thymidine phosphorolysis.