Increased respiration in the sch9 mutant is required for increasing chronological life span but not replicative life span

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Loss of the protein kinase Sch9p increases both the chronological life span (CLS) and the replicative life span (RLS) of Saccharomyces cerevisiae by mimicking calorie restriction, but the physiological consequences of SCH9 deletion are poorly understood. By transcriptional profiling of an sch9 mutant, we show that mitochondrial electron transport chain genes are upregulated. Accordingly, protein levels of electron transport chain subunits are increased and the oxygen consumption rate is enhanced in the sch9 mutant. Deletion of HAP4 and CYT1, both of which are essential for respiration, revert the sch9 mutant respiratory rate back to a lower-than-wild-type level. These alterations of the electron transport chain almost completely blocked CLS extension by the sch9 mutation but had a minor impact on the RLS. SCH9 thus negatively regulates the CLS and RLS through inhibition of respiratory genes, but a large part of its action on life span seems to be respiration independent and might involve increased resistance to stress. Considering that TOR1 deletion also increases respiration and that Sch9p is a direct target of TOR signaling, we propose that SCH9 is one of the major effectors of TOR repression of respiratory activity in glucose grown cells.

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