Regulation of nuclear genes encoding mitochondrial proteins in Saccharomyces cerevisiae

Abstract

Selection for mutants which release glucose repression of the CYB2 gene was used to identify genes which regulate repression of mitochondrial biogenesis. We have identified two of these as the previously described GRR1/CAT80 and ROX3 genes. Mutations in these genes not only release glucose repression of CYB2 but also generally release respiration of the mutants from glucose repression. In addition, both mutants are partially defective in CYB2 expression when grown on nonfermentable carbon sources, indicating a positive regulatory role as well. ROX3 was cloned by complementation of a glucose-inducible flocculating phenotype of an amber mutant and has been mapped as a new leftmost marker on chromosome 2. The ROX3 mutant has only a modest defect in glucose repression of GAL1 but is substantially compromised in galactose induction of GAL1 expression. This mutant also has increased SUC2 expression on nonrepressing carbon sources. We have also characterized the regulation of CYB2 in strains carrying null mutations in two other glucose repression genes, HXK2 and SSN6, and show that HXK2 is a negative regulator of CYB2, whereas SSN6 appears to be a positive effector of CYB2 expression.When Saccharomyces cerevisiae strains are grown on glucose, they down regulate the biogenesis of mitochondria such that the volume occupied by mitochondria is decreased from 12 to 14% of the total cellular volume under derepressing conditions to 3 to 4% under repressing conditions. The glucose-repressed mitochondria also have poorly differentiated inner membrane cristae (51). Although this effect of glucose on yeast mitochondria was discovered over 40 years ago (14), surprisingly little is known about the mechanism of glucose repression of mitochondrial biogenesis.Much of what we know about glucose repression is from studies of the SUC and GAL genes, which are involved in the regulation of sucrose and galactose metabolism, respectively (27). Some genes which regulate repression of sucrose and galactose catabolic enzymes also regulate at least some mitochondrial enzymes. For example, mutations in HXK2, which encodes one of the hexokinases, release repression of mitochondrial cytochrome c reductase and cytochrome c oxidase activities (38). Mutations in CYC8 (SSN6) and CYC9 (TUP1) both cause overexpression of iso-2-cytochrome c (46), and mutations in GRR1 and CAT4 allow iso-1-cytochrome c to escape glucose repression (17, 48). However, the extent to which these genes may be general regulators of mitochondrial biogenesis remains unknown.Boker-Schmitt and coworkers isolated two constitutively highly respiring mutants, CCR-91 and CCR-96, in which respiration, mitochondrial cytochromes, and mitochondrial DNA synthesis were released from glucose repression (2). Significantly, the activities of maltase and malate synthase, two nonmitochondrial glucose-regulated enzymes, were not changed in the CCR-91 mutant, indicating that glucose repression of mitochondrial biogenesis is at least partially independent of repression of enzymes involved in disaccha...

Med19(Rox3) Regulates Intermodule Interactions in the Saccharomyces cerevisiae Mediator Complex

“…amino acids of the C terminus of the 220-amino acid protein (5,21,23). The placement of Med19(Rox3) in this model and its genetic interactions (16,18,19) are more consistent with it being characterized as a Middle module subunit (Fig.…”

“…An additional possibility is that the absence of Med19(Rox3) might also lead to a stabilization of the proposed Head-Tail interaction, even while it destabilizes the interaction of the Middle module with the Head-Tail complex. Med19(Rox3) exerting both stabilizing and destabilizing effects on the complex would be consistent with its dual role in both activation and repression observed in the cell (16,18,19,23,24).…”

“…Med19(Rox3) was initially found to be encoded by an essential gene product; however, a strain made as part of the genome wide deletion library lacks MED19(ROX3) and is viable (EUROSCARF). med19(rox3) mutants have been shown to be defective for activation of Gal4 (23) and Gcn4 (24)-induced genes. On the other hand, med19(rox3) mutants also lead to derepression of heat shock (16), glucose-repressed (18), and HO genes (19).…”

A Functional Module of Yeast Mediator That Governs the Dynamic Range of Heat-Shock Gene Expression

“…For example, the yeast Gal11 and Rox3 subunits are required for Gal4 activation of galactose-inducible genes (Suzuki et al 1988;Brown et al 1995), Nut2 for Gcn4-mediated activation of amino acid biosynthetic genes (Han et al 1999), and Med11 for MFa2 transcriptional activation (Han et al 1999). Similar activator-specific functions of individual subunits have been described for Drosophila Mediator, including a role for MED23 in heat-shock gene expression and MED16 in lipopolysaccharide gene expression (Kim et al 2004).…”

In Vivo Requirement of Activator-Specific Binding Targets of Mediator

“…Even though the Mediator complex as a whole is required in general for Pol II transcription, some of its subunits function in an activator-specific manner to modulate the expression of a distinct subset of genes (13,15,23). In addition, genetic evidence suggests that a subset of the Mediator proteins are involved in transcriptional repression (4,8,17,35). Differential dissociation of the Mediator components by high-urea treatment (22) and compositional analysis of mutant hpol II complexes (23,24,26) revealed that Mediator subunits with similar genetic properties form distinct modular subassemblies.…”