Studies on a Temperature-sensitive Irreparable Mutant of Neurospora crassa. II

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Abstract
The temperature-sensitive irreparable mutant b39a of Neurospora crassa was obtained from the microconidial prototrophic strain ma after nitrogen mustard treatment of the microconidia. The mutant character of b39a is due to a single gene which is located on the mating type chromosome. At 23[degree]C in a minimal medium the mutant grows like the parent strain, but it cannot grow at 34[degree]C even in complete media. At 26[degree] - 28 [degree]C b39a grows slowly showing its mutant character. The growth of b39a at 26[degree]C was markedly inhibited when cultivated in the medium containing 10-4 M L-histidine, whereas the growth of the parent was not inhibited by 2x10-2 M L-histidine, except in early stages. The histidine inhibition was reversed competitively by glycine, L-methionine and other amino acids. Intracellular free amino acids were determined, but unbalanced syntheses of these amino acids were not detected. It was shown later that this inhibition was also reversed, again competitively by higher concentrations of glucose or of a nitrogen source. It seemed, therefore, that the sensitivity og b39a to L-histidine is due to intensification of a defect in a general metabolic process, and is not due to an unblance of amino acid metabolism. The growth of the mutant is favorable in a medium containing isotonic concentrations of polyethylene glycol, NaCl and other substances. In these media, an outflow of substances positive to phenol reagent was prevented. It was suggested that the temperature-sensitive irreparable mutant strain b39a has a defect in its cell membrane or in a transport system, resulting in that substances of importance in cell metabolism are liable to flow out of the cell.

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