A study of cold-sensitive mutants in Aspergillus nidulans .

Mutants of A. nidulans which grow normally at 37°C but not at 20°C have been isolated. They are designated cs (cold-sensitive). Growth tests of 75 cs mutants demonstrate that they have a range of defects. Genetic analysis of some of these strains shows that mutation to cold-sensitivity can arise in many loci and affect many functions. Two of the cs mutants (cs 13 and cs 48) have a specific nutritional requirement for growth at 20°C (isoleucine and choline respectively). One strain, cs 67, exhibits extranuclear inheritance of cold-sensitivity. After growth at 37°C followed by incubation at 20°C, four of the cs mutants produce altered ribosome sedimentation profiles. These strains are designated arp and all four produce profiles containing a reduced ratio of the amounts of large to small ribosomal subunits. In at least three of the arp strains the altered profile results from decreased production of large ribosomal subunits at 20°C. In each of the arp strains there is a single mutation which determines both cold-sensitivity and production of an altered ribosome profile. These four mutations define three loci, one of which (arp A) is in linkage group VII. The other two loci (arp B and arp C) are in linkage group VIII but are not closely linked. Both mutations at the arp A locus also confer resistance to a high concentration of actidione. Other mutants were selected for actidione-resistance and their analysis has identified a further six loci which determine resistance to this drug. The properties of the arp strains are compared with those of mutants affecting ribosome synthesis in bacteria and in other eukaryotes.