Discrimination between bromouracil and thymine for uptake into DNA in drm- and dra- mutants of Escherichia coli K12.

The relative efficiency of bromouracil and thymine for uptake into DNA was measured in various thymine-requiring strains of Escherichia coli K12. It was found that: 1. Mutants with genotype thyA- dra- discriminate against bromouracil to much greater extent than do mutants with genotype thyA- drm-. 2. The discrimination in dra-mutants is dependent on thymine concentration, whereas discrimination in drm- mutants is almost independent of thymine concentration. It is suggested that the intracellular level of deoxyribose 5-phosphate affects the efficiency of uptake into DNA of bromouracil relative to thymine.