The presence and distribution of reduced folates in Escherichia coli dihydrofolate reductase mutants.

Escherichia coli DNA photolyase was overproduced and purified from each of two mutant E. coli strains lacking dihydrofolate reductase. The extent of over-production in the mutants was comparable to that seen in the wild type strain. Examination of the isolated photolyase from these strains revealed that the folate cofactor, 5,10-methenyltetrahydrofolate, was present in these proteins at a level of 60-80% compared to that purified from the wild type strain. Further examination of the dihydrofolate reductase-deficient strains revealed the presence of other tetrahydrofolate derivatives. These findings demonstrate that dihydrofolate reductase is not essential for the production of tetrahydrofolates in E. coli.