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

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Schizosaccharomyces pombegenome contains an essential genehmg1+ encoding the sterol biosynthetic enzyme, 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR). Here, we isolated an allele of thehmg1+ gene,hmg1-1/its12, as a mutant that showed sensitivities to high temperature and to FK506, a calcineurin inhibitor. Thehmg1-1allele 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 thehmg1-1mutant gene suppressed the mutant phenotypes. Thehmg1-1mutant 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.

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