Pleiotropic phenotypes caused by an opal nonsense mutation in an essential gene encoding HMG-CoA reductase in fission yeast.

Schizosaccharomyces pombe genome contains an essential gene hmg1(+) encoding the sterol biosynthetic enzyme, 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR). Here, we isolated an allele of the hmg1(+) gene, hmg1-1/its12, as a mutant that showed sensitivities to high temperature and to FK506, a calcineurin inhibitor. The hmg1-1 allele contained an opal nonsense mutation in its N-terminal transmembrane domain, yet in spite of the mutation a full-length protein was produced, suggesting a read-through termination codon. Consistently, overexpression of the hmg1-1 mutant gene suppressed the mutant phenotypes. The hmg1-1 mutant showed hypersensitivity to pravastatin, an HMGR inhibitor, suggesting a defective HMGR activity. The mutant treated with FK506 caused dramatic morphological changes and showed defects in cell wall integrity, as well as displayed synthetic growth phenotypes with the mutant alleles of genes involved in cytokinesis and cell wall integrity. The mutant exhibited different phenotypes from those of the disruption mutants of ergosterol biosynthesis genes, and it showed normal filipin staining as well as showed normal subcellular localization of small GTPases. These data suggest that the pleiotropic phenotypes reflect the integrated effects of the reduced availability of ergosterol and various intermediates of the mevalonate pathway.