Genetic Control of Biochemical Reactions in Neurospora

Abstract

Para-aminobenzoic acid has recently been recognized as a factor required for the growth of a number of micro6rganisms' and as a member of the vitamin B group.2One of the number of x-ray induced mutants of Neuro- spora crassa, obtained as described elsewhere,' is characterized by the loss of ability to synthesize p-aminobenzoic acid.This "aminobenzoicless" mutant is differentiated from normal by a single gene, is unable to grow on unsupplemented synthetic medium, but its growth is indistinguishable from normal when p-aminobenzoic acid is supplied.The mutant culture was maintained on "complete" medium containing yeast extract, malt extract and sucrose.Conidia from this stock culture were used for inoculating test cultures.The test medium was the "synthetic" or "m nimal" medium previously described8 containing inorganic salts and nitrogen, sucrose and 4 gammas of biotin4 per 1000 ml.Agar used in this synthetic medium was freed of p-aminobenzoic acid by repeated washing with distilled water.Cultures were incubated at 25°C.Crosses were made of the p-aminobenzoicless mutant (sex A, the equiva- lent of + in Lindegren's terminology5) with the normal strain (sex a); ascospores were dissected in order from asci and grown on complete me- dium.These cultures were tested by transferring them to minimal me- dium and observing for growth.In all, 142 single spore cultures were grown and tested.Of these, 72 proved to be able to synthesize p-amino- benzoic acid and 70 were unable to accomplish this.Of 11 asci from each of which all eight ascospore cultures were obtained, all had four normal