Growth and macromolecular synthesis phenotypes of a heat-sensitive mutant strain, rip-1, of Neurospora crassa.

A heat-sensitive mutant of Neurospora crassa, strain 4M(t), was isolated using ultraviolet-light mutagenesis followed by the inositol-less death enrichment technique. The heat-sensitivity is the result of a single gene mutation which maps to the distal end of the right arm of linkage group II. The mutation defines the rip-1 gene locus. Both conidial germination and mycelial extension are inhibited in the mutant at 35 degrees C and above (the nonpermissive temperature) but prolonged incubation at that temperature is not lethal to either cell type. Analysis of the lateral mycelial growth rates of wild type and of the rip-1 mutant at a variety of temperatures between 10 and 40 degrees C indicated that the maximal growth rate occurs at 35 degrees C in the wild type, and at 25 degrees C in the rip-1 strain. The rip-1 mutant grows 239-times slower at 35 degrees C than at 25 degrees C, whereas the wild type grows 1.4-times faster. Temperature shift-up experiments showed that even 3 h at 20 degrees C is not sufficient to allow germination at 37 degrees C, thereby showing that the mutant cannot accumulate enough heat-sensitive product at the permissive temperature to contribute to germination at 37 degrees C. The reciprocal temperature shift-down experiments showed that the molecular events at 37 degrees C may be qualitatively useful for germination after shifting to 20 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)