mutation causes significant changes in expression of 643 genes, including a systematic increase in mitochondrial respiratory chain gene expression. Here, we have shown that mitochondrial activity of the sch9Δ
mutant is increased. This finding was previously reported for CR yeast and in the tor1Δ
Increasing respiration has drawbacks for cellular physiology by producing detrimental by-products such as reactive oxygen species (ROS) that limit the life span of cells in stationary-phase cultures, likely by oxidizing proteins, lipids, and nucleic acids (16
). Besides ROS, ethanol also limits the CLS of yeast cells (9
). Previous work has shown that the sch9Δ
deletion causes an increase in heat and oxidative stress resistance, and SOD2
was reported to be a downstream effector of the sch9Δ
deletion increase in oxidative stress resistance and CLS (10
). Our expression profiling experiments show that many stress response genes are upregulated in sch9Δ
cells (Table ): we confirm increased expression of the mitochondrial superoxide dismutase gene SOD2
and of several heat shock protein genes, and we show increased transcription of ethanol-degrading enzyme genes ADH1
, and -3
. This provides additional molecular evidence for the sch9Δ
mutant's resistance to stress and accelerated ethanol depletion (9
Stress response genes and other metabolic genes that are significantly modulated in the sch9Δ mutant (P < 0.05)
Our study shows that not only is the sch9Δ mutant a genetic mimic of CR, but it also recapitulates many molecular hallmarks of CR in yeast. In fact, inactivation of SCH9 provokes shunting of the fermentative metabolism of yeast to a more respirative mode and promotes the heat shock response and the turnover of ROS and alcohol. We find that a large part of the SCH9 effect on the CLS is mediated by respiration, since deletion of HAP4 or CYT1 extensively abrogated sch9Δ-dependent extension of the CLS while RLS extension of sch9Δ was only slightly reduced by blocking respiration. Therefore, our data suggest that the sch9Δ mutation influences the CLS and RLS by both respiration-dependent and -independent mechanisms. Our data also indicate that the conditions required for extending the CLS and RLS are probably encountered by different means in the sch9Δ mutant, since disrupting respiration has a more dramatic effect on the CLS than on the RLS.
Our data support a recent finding that has challenged the relationship between CR-induced RLS extension and respiration (20
). Those authors demonstrated that CR causes RLS extension in respiration-deficient yeast strains. Since sch9Δ
is thought to be a CR genetic mimic and since it does not fully rely on respiration to promote CLS and RLS, our work further weakens the evidence linking CR-induced RLS and increased respiration.
The respiration-independent effect of the sch9Δ mutation might be explained by it being resistant to oxidative and heat stress through increased ROS and ethanol turnover, which we detect in expression analysis of exponentially growing cultures. Thus, reduced production of detrimental by-products during the growth phase caused by a respirative metabolism and faster turnover of ethanol and ROS might lead to increased survival of the sch9Δ mutant during CLS analysis.
It is believed that Sch9p, Tor1p, and PKA kinases belong to highly integrated signaling pathways that are all involved in nutrient sensing (19
inhibition increase CLS and RLS and are thought to mimic CR (23
). Recent findings show that Sch9p is the direct target of the TORC1 complex and that the tor1Δ
strains display increased CLS and respiration (4
). This, in addition to our findings, supports the view that the TOR-SCH9
pathway feeds into the regulation of respiration and that Sch9p might be one of the major effectors of TOR repression of respiratory activity, as it is the major effector of translational activation by the TOR pathway. The CCAAT box-binding complex (Hap2/3/4/5p) is a likely downstream effector of this nutrient-dependent signal transduction pathway. Hap4p was originally proposed to be the regulatory moiety of the CCAAT box-binding complex, and the TOR
pathway could thus reduce respiratory chain expression in high-glucose conditions by repressing Hap4p by a direct or indirect mechanism; in contrast, when glucose is depleted, decreased Sch9 signaling would induce the respiratory regulon as observed in our sch9Δ
It has been proposed that one of the major mitochondrial targets of TOR is translation (4
). Here, we suggest that the TOR-SCH9
pathway not only impinges on mitochondrial translation but also affects transcriptional activity of the nuclear respiratory regulon through Sch9p.