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A method was developed for the isolation of Neurospora bradytrophs. The bradytrophs (representing lesions in a number of pathways) were resistant to DL-p-fluorophenylalanine when growing in a leaky fashion but were sensitive when grown in the presence of their stimulating supplement.

A procedure has been developed for isolating nicotinamide adenine dinucleotide phosphate-specific glutamate dehydrogenase (am) mutants of Neurospora. Physiological, genetic, and enzymatic tests show that the new mutants are am alleles. Reconstruction tests and analysis of the new alleles show that the procedure yields a broad spectrum of lesions at the am locus. The isolation of am mutants by this technique appears to be related to the effect of am mutants on the control of the general permease.

A tryptophan auxotroph of Neurospora crassa, trp-5, has been characterized as a mutant with a deficient tryptophanyl-transfer ribonucleic acid (tRNA) synthetase (EC 6.1.1.2) activity. When assayed by tryptophanyl-tRNA formation, extracts of the mutant have less than 5% of the wild-type specific activity. The adenosine triphosphate-pyrophosphate exchange activity is at about half the normal level. In the mutant derepressed levels of anthranilate synthetase and tryptophan synthetase were associated with free tryptophan pools equal to or higher than those found in the wild type. We conclude that a product of the normal tryptophanyl-tRNA synthetase, probably tryptophanyl-tRNA, rather than free tryptophan, participates in the repression of the tryptophan biosynthetic enzymes.

A new p-fluorophenylalanine resistant mutant of Neurospora (fpr-1) was isolated. It is unaffected by the suppressor of a previously described resistance mutant, mtr. The fpr-1 locus is on linkage group V, tightly linked to act-2. The expression of resistance to p-fluorophenylalanine by fpr-1 can be suppressed by genes controlling a requirement for lysine or arginine. The suppression seems to involve an increased sensitivity of the lysine and arginine auxotrophs to p-fluorophenylalanine.