FOLIC ACID METABOLISM IN ANTIFOLIC-RESISTANT MUTANTS OF STREPTOCOCCUS FAECALIS.

Johnson, Alva H. (Sloan-Kettering Institute for Cancer Research, Rye, N.Y.), and Dorris J. Hutchison. Folic acid metabolism in antifolic-resistant mutants of Streptococcus faecalis. J. Bacteriol. 87:786-791. 1964.-Streptococcus faecalis ATCC 8043 and three of its mutants resistant to amethopterin were compared for their quantitative requirements for serine, purines, and thymine; for their quantitative requirements of folic acid (FA) for the synthesis de novo of serine, purines, and thymine; for the susceptibility to amethopterin of each pathway; and for the relative capacity of resting cells of each culture to synthesize N(5)-formyltetrahydrofolate (5-CHOFAH(4)) from FA and serine or FA and formate. The mutants were found to be both qualitatively and quantitatively different from one another and from the wild strain. The growth conditions, specifically the composition of the medium in which each mutant strain was selected, had a marked effect on the metabolic capacities of the mutants. The ability to synthesize serine, purines, and thymine, as observed from the FA requirements, directly reflected the level of resistance of each pathway to amethopterin. The resistant mutants were more efficient than the wild strain in the formation of 5-CHOFAH(4) from FA and formate and, furthermore, this formate activation paralleled their capacities in the synthesis de novo of serine. Alterations in purine and thymine biosyntheses were also observed.