Studies on a Temperature-sensitive Irreparable Mutant of <i>Neurospora crassa</i>. II

Abstract

A temperature-sensitive irreparable mutant b39a of Neurospora crassa was studied genetically, biochemically and physiologically. The mutant character of b39a is due to the mutation of a single gene which is located on the mating type chromosome. At 23°C in minimal medium the mutant grows as well as does the parent strain, but it cannot grow at 34°C even in complete media. At 26°-28°C b39a grows slowly showing its mutant character.The growth of b39a at 26°C was markedly inhibited when it was cultivated in the medium containing 10-4M L-histidine, whereas growth of the parent was not inhibited by as much as 2×10-2M 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 seems, therefore, that the sensitivity of b39a to L-histidine is due to intensification of a defect in a general metabolic process, and is not due to an unbalance of amino acid metabolism.The effect of hypertonic medium on the growth of b39a is suggestive. Growth of the mutant is favored in a medium containing isotonic concentrations of polyethylene glycol, NaCl and other substances. In these media, the outflow of substances positive to phenol reagent was prevented.These data suggest that the temperature-sensitive irreparable mutant strain b39a has a defect in its cell membrane or in a transport system, of such a nature that substances of importance in cellular metabolism are liable to flow out of the cell.