Heat-sensitive mutant strain of Neurospora crassa, 4M(t), conditionally defective in 25S ribosomal ribonucleic acid production

Characterization of the temperature-sensitive mutations un-7 and png-1 in Neurospora crassa

The model filamentous fungus Neurospora crassa has been studied for over fifty years and many temperature-sensitive mutants have been generated. While most of these have been mapped genetically, many remain anonymous. The mutation in the N. crassa temperature-sensitive lethal mutant un-7 was identified by a complementation based approach as being in the open reading frame designated NCU00651 on linkage group I. Other mutations in this gene have been identified that lead to a temperature-sensitive morphological phenotype called png-1. The mutations underlying un-7 result in a serine to phenylalanine change at position 273 and an isoleucine to valine change at position 390, while the mutation in png-1 was found to result in a serine to leucine change at position 279 although there were other conservative changes in this allele. The overall morphology of the strain carrying the un-7 mutation is compared to strains carrying the png-1 mutation and these mutations are evaluated in the context of other temperature-sensitive mutants in Neurospora.

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)