The sulfonamide-requiring mutant of Neurospora: threonine-methionine antagonism

The present paper reports experiments that partially identify the reaction responsible for the sulfonamide-requiring character. From studies previously reported it has been supposed that there is a gene-controlled deleterious reaction (in sfo strains) that requires relatively higher levels of PABA than are needed for other PABA-dependent metabolic reactions. The present studies show that the deleterious reaction also requires somewhat more methionine (or homocysteine) than the minimum required for normal growth and that the inhibition resulting from excess methionine is competitively antagonized by threonine (or homoserine).