Yeast mitochondrial transcript and gene product abundance has been observed to increase upon release from glucose repression, but the mechanism of regulation of this process has not been determined. We report a kinetic analysis of this phenomenon, which demonstrates that the abundance of all classes of mitochondrial RNA changes slowly relative to changes observed for glucose-repressed nuclear genes. Several cell doublings are required to achieve the 2-to 20-fold-higher steady-state levels observed after a shift to a nonrepressing carbon source. Although we observed that in some yeast strains the mitochondrial DNA copy number also increases upon derepression, this does not seem to play the major role in increased RNA abundance. Instead we found that three-to sevenfold increases in RNA synthesis rates, measured by in vivo pulse-labelling experiments, do correlate with increased transcript abundance. We found that mutations in the SNF1 and REG] genes, which are known to affect the expression of many nuclear genes subject to glucose repression, affect derepression of mitochondrial transcript abundance. These genes do not appear to regulate mitochondrial transcript levels via regulation of the nuclear genes RP041 and MTF1, which encode the subunits of the mitochondrial RNA polymerase. We conclude that a nuclear gene-controlled factor(s) in addition to the two RNA polymerase subunits must be involved in glucose repression of mitochondrial transcript abundance.Mitochondria are the site of many complex biochemical processes, including the citric acid cycle, oxidative phosphorylation, and electron transport (reviewed in reference 1). In addition to these processes, the organelles maintain their own genomes and transcribe and synthesize the proteins they encode (reviewed in references 7 and 33). In addition to being dependent on the organelle-encoded gene products, mitochondria are functionally dependent upon many genes encoded in the nuclear genome for all of these processes. Mitochondrial activity in the yeast Saccharomyces cerevisiae is subject to regulation; under repressing conditions, such as anaerobic growth or growth on glucose, mitochondrial activity is minimal. However, under derepressing conditions, such as growth on a nonfermentable carbon source, mitochondrial activity is high and the requirement for mitochondrial enzymes increases (7,26). Since expression of the mitochondrial genes is dependent on nuclear genes, coordinated regulation of expression of the nuclear and mitochondrial genomes must be involved in these changes.It is well documented that glucose repression is a major regulatory system in yeast cells and affects a large number of nuclear genes and gene products (reviewed in references 35 and 36). It has also been shown that glucose repression affects the mitochondrion in several ways. The expression of many nucleus-encoded mitochondrial genes is glucose repressed at the levels of transcription and RNA stability (reviewed in references 20, 35, and 36). pressed cells (reviewed in references 7 and 33)...

We have found that many laboratory strains of yeast are defective in galactose metabolism owing to a recessive mutation in the previously characterized nuclear gene, IMP1. This defect leads to a requirement for mitochondrial function for growth on, and metabolism of, galactose. Genetic background affects the degree to which cells are defective. In particular, alleles of GAL3 affect the ability to score the Imp phenotype. We have found that in imp1 strains, transcriptional induction of the galactose inducible genes (GAL1, 2, 7 + 10, MEL1) is normal, but galactose transport is reduced in both rho+ and rho0 cells. This phenotype is normally associated with mutations in GAL2, the galactose permease. Although the growth phenotypes of gal2 and imp1 mutants are distinct, we found that the transformation of imp1 rho0 strains with a plasmid containing the GAL2 gene allows these strains to grow on galactose. Initial genetic analyses did not demonstrate linkage between the GAL2 and IMP1 genes owing to the effects of an unlinked gene on the Imp phenotype. By disrupting the GAL2 gene in an Imp+ background, we have shown that IMP1 and GAL2 segregate as tightly linked genes. Based on these data, we believe that imp1 is a partially defective allele of the GAL2 gene.

MTF1 is a nuclear gene that encodes the promoter recognition factor of the yeast mitochondrial RNA polymerase. The MTF1 gene was physically mapped to chromosome XIII. Genetic mapping data indicate that the gene is closely linked to RNA1.

Yeast mitochondrial transcript and gene product abundance has been observed to increase upon release from glucose repression, but the mechanism of regulation of this process has not been determined. We report a kinetic analysis of this phenomenon, which demonstrates that the abundance of all classes of mitochondrial RNA changes slowly relative to changes observed for glucose-repressed nuclear genes. Several cell doublings are required to achieve the 2- to 20-fold-higher steady-state levels observed after a shift to a nonrepressing carbon source. Although we observed that in some yeast strains the mitochondrial DNA copy number also increases upon derepression, this does not seem to play the major role in increased RNA abundance. Instead we found that three- to sevenfold increases in RNA synthesis rates, measured by in vivo pulse-labelling experiments, do correlate with increased transcript abundance. We found that mutations in the SNF1 and REG1 genes, which are known to affect the expression of many nuclear genes subject to glucose repression, affect derepression of mitochondrial transcript abundance. These genes do not appear to regulate mitochondrial transcript levels via regulation of the nuclear genes RPO41 and MTF1, which encode the subunits of the mitochondrial RNA polymerase. We conclude that a nuclear gene-controlled factor(s) in addition to the two RNA polymerase subunits must be involved in glucose repression of mitochondrial transcript abundance.