Provides the tetrahydrofolates necessary for the synthesis of nucleotides and amino acids. Bacteriophage T5 induces high levels of dihydrofolate reductase in the host cell, probably for the viral replication. (6S)-5,6,7,8-tetrahydrofolate + NADP(+) = 7,8-dihydrofolate + H(+) + NADPH Expressed in the early phase of the viral replicative cycle.