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Present knowledge of the function of para-aminobenzoic acid (PABA) has been derived almost exclusively from sulfanilamide (SAN) inhibition studies. Shive and Roberts (1946) have advanced the hypothesis that PABA is catalytically active in the synthesis of a number of metabolic products in Escherichia coli and that the enzyme systems concerned in their production are differentially sensitive to SAN inhibition. They present evidence that the most sensitive reaction to SAN is that concerned with methionine synthesis, and adenine synthesis is the next most sensitive step. However, since methionine and adenine are incapable of reversing SAN inhibition in all concentrations at which PABA was effective, it was concluded that PABA is concerned in other unknown synthetic reactions. Later, Winkler and de Haan (1948) reported that, in the presence of methionine and adenine, serine and pteroylglutamic acid (or thymine) would reverse the inhibitory effects of higher concentrations of SAN. The results indicate that the latter compounds are also products of reactions in which PABA is concerned. Strandskov (1947), in a further study of PABA antagonists, found that the inhibition of growth of E. coli by 2-chloro-PABA could be reversed by methionine at any concentration of the inhibitor. Strains that became resistant to 2-Cl-PABA were shown, however, not to be resistant to SAN, and from these data Strandskov concluded that SAN inhibits a step in methionine synthesis in addition to that inhibited by 2-Cl-PABA. The present report is concerned with the characteristics of a biochemical mutant of Neurospora whose growth requirements can be satisfied by either methionine or PABA. The response of this mutant to various growth factors in the presence and absence of SAN has been studied. The results indicate that one of the rate-limiting steps in the growth of this organism is the methylation of homocysteine to form methionine.

THE REPLACEMENT OF PARA-AMINOBENZOIC ACID BY METHIONINE IN THE GROWTH OF A NEUROSPORA MUTANT