Viability offoL4-Null Derivatives of Wild-Type (thyA+) Escherichia coli K-12

Dihydrofolate reductase (thefolA gene product) catalyzes the synthesis of tetrahydrofolate, a key methyl donor in many biosynthetic pathways. Loss of folA had been thought to be lethal to wild-type (thyA+) Escherichia coli. Viablefoi4-null derivatives of thyA+ E. coli were obtained, however, by recombining afolA deletion linked to a Kanr selectable marker into a AJoL4 phage and using this phage to transduce cells to kanamycin resistance. foUL-null strains were slow growing, formed small colonies, and were auxotrophic for thymidine, adenine, methionine, glycine, and pantothenate. Dihydrofolate reductase (thefolA gene product) catalyzes the synthesis of tetrahydrofolate (FH4), which is needed for the syntheses of thymidine, adenine, methionine, glycine, pantothenate, and N-formyl methionine tRNA. foL4-null mutant derivatives of Escherichia coli had been obtained in thyA mutant, but not in wild-type (thyA+), strains in earlier allelic replacement experiments, which had suggested that foU is essential unless a compensating mutation is present (1, 3). thyA mutants are blocked in the conversion of dUM