Prion-dependent switching between respiratory competence and deficiency inthe yeast nam9-1 mutant

Nam9p is a protein of the mitochondrial ribosome. The respiration-deficient Saccharomyces cerevisiae strain MB43-nam9-1 expresses Nam9-1p containing t he point mutation S82L. Respiratory deficiency correlates with a decrease i n the steady level of some mitochondrially encoded proteins and the complet e lack of mitochondrially encoded cytochrome oxidase subunit 2 (Cox2). De n ovo synthesis of Cod in MB43-nam9-1 is unaffected, indicating that newly sy nthesized Cox2 is rapidly degraded. Respiratory deficiency of MB43-nam9-1 i s overcome by transient overexpression of HSP104, by deletion of HSP104, by transient exposure to guanidine hydrochloride, and by expression of the C- terminal portion of Sup35, indicating an involvement of the yeast prion [PS I+]. Respiratory deficiency of MB43-nam9-1 can be reinduced by transfer of cytosol from S. cerevisiae that harbors [PSI+]. We conclude that nam9-1 cau ses respiratory deficiency only in combination with the cytosolic prion [PS I+], presenting the first example of a synthetic effect between cytosolic [ PSI+] and a mutant mitochondrial protein.