Growth Responses of a Sulfonamide-requiring Mutant Strain of Neurospora

A mutant strain of Neurospora crassa has appeared in which the antagonistic roles of p-aminobenzoic acid and the sulfonamides have been reversed to a considerable extent. Optimal growth of this strain occurs only in the presence of sulfonamides. Conversely, p-aminobenzoic acid is a potent fungistatic agent for this strain under certain conditions. To say that sulfanilamide has become an essential metabolite and p-aminobenzoic acid an inhibiting analog would be to oversimplify the altered physiology of this mutant strain. It will be shown that, in this strain, both sulfonamides and p-aminobenzoic acid are essential for growth, and that each acts as an inhibiting analog of the other. These interrelations are further complicated by the effect of temperature on the need for sulfonamides, and on the inhibition by p-aminobenzoic acid. The present report deals exclusively with the growth responses of this mutant strain to sulfonamides, to temperature, and to p-aminobenzoic acid. At the present time nothing definite is known of the physiological role of sulfonamides in this strain.