Differential regulation of the two genes encoding Saccharomyces cerevisiae cytochrome c oxidase subunit V by heme and the HAP2 and REO1 genes.

Abstract

In Saccharomyces cerevisiae, the COXSa and COXSb genes encode two forms of cytochrome c oxidase subunit V, Va and Vb. We report here that heme increases COX5a expression and decreases COX5b expression and that the HAP2 and REOI genes are involved in positive regulation of COX5a and negative regulation of COX5b, respectively. Heme regulation of COX5a and COX5b may dictate which subunit V isoform is available for assembly into cytochrome c oxidase under conditions of high-and low-oxygen tension.The recent discovery of different forms (isologs) for some of the nuclear-encoded subunits of cytochrome c oxidase in mammals and yeasts has led to the hypothesis that isologs play a role in altering the functional properties of the holoenzyme (2,9,10,21 The effect of heme deprivation on expression of COXSa and COX5b was tested with strains carrying hem] mutations. hemi mutant strains are heme deficient because of an inability to synthesize b-amino levulinic acid (8-ALA) (7, 23); they can be made heme proficient by the addition of b-ALA to the growth medium. RNA blot analysis (Fig. 1) demonstrated that a hemi mutant strain grown in the presence of b-ALA (lanes 1 and 3) had increased steady-state levels of COXSa transcripts and decreased levels of COX5b transcripts compared with transcript levels of the same strain grown in the absence of b-ALA (lanes 2 and 4).Additional evidence for inverse regulation of COXSa and COX5b by heme was provided by ,-galactosidase activities measured in hemi mutant strains transformed with COXSalacZ and COX5b-lacZ fusion genes, carried on the 2,umbased plasmids pCT5aL and pMC5bL, which were constructed as described previously (21). Cultures carrying these plasmids were grown to exponential phase in synthetic glucose medium (2) lacking uracil (to select for the plasmids) but containing 40 ,ug of histidine, 40 CYCI (8,15,16). The hapl::LEU2 mutation resulted in a small decrease (about 1.5-fold) in COXSa-lacZ expression and had little or no effect on COXSb-lacZ expression in aerobically grown heme-proficient cells. The significance of these small changes is unclear. In contrast, the hap2-1 mutation in strain LGW1 had a dramatic effect on expression of COXSa-lacZ, reducing it about eightfold relative to expression in the isogenic HAP2+ strain, BWG1-7A, but had no effect on the expression of COXSb-lacZ (

Different subcellular localization of Saccharomyces cerevisiae HMG-CoA reductase isozymes at elevated levels corresponds to distinct endoplasmic reticulum membrane proliferations.

“…Similar to HMG1 and HMG2, subunit V of the yeast mitochondrial cytochrome c oxidase complex and cytochrome c are encoded by different isoforms whose abundance changes depending on the cellular levels of heme (Guarente and Mason, 1983;Laz et al, 1984;Trueblood et al, 1988). It has been suggested that the more abundant subunits under low oxygen conditions are better adapted to those conditions and possibly to interactions with each other (Trueblood et al, 1988). Likewise, Hmglp may be better suited for aerobic conditions and Hmg2p for low oxygen conditions.…”

“…The differential expression of HMG1 versus HMG2 under different oxygen levels acts as a buffer so that changing levels of heme do not affect the total levels of HMGR in the cell (Thorsness et al, 1989). Similar to HMG1 and HMG2, subunit V of the yeast mitochondrial cytochrome c oxidase complex and cytochrome c are encoded by different isoforms whose abundance changes depending on the cellular levels of heme (Guarente and Mason, 1983;Laz et al, 1984;Trueblood et al, 1988). It has been suggested that the more abundant subunits under low oxygen conditions are better adapted to those conditions and possibly to interactions with each other (Trueblood et al, 1988).…”

LEAFY COTYLEDON1-LIKE Defines a Class of Regulators Essential for Embryo Development

“…CBFs are eukaryotic transcriptional activators that serve diverse roles in different organisms (Li et al, 1992). In yeast, CBF activates a set of genes involved in mitochondrial respiration (Guarente et al, 1984;Keng and Guarente, 1987;Trueblood et al, 1988;Schneider and Guarente, 1991), whereas mammalian CBFs are thought to act generally to enhance transcription rates, often in combination with other proteins (reviewed by Maity and De Crombrugghe, 1998;Mantovani, 1999). The transcription factor is a hetero-oligomeric complex consisting of at least three subunits, HAP2, HAP3, and HAP5, although yeast possesses a fourth subunit, HAP4 (reviewed by Maity and De Crombrugghe, 1998;Mantovani, 1999).…”

Oxygen-regulated isoforms of cytochrome c oxidase have differential effects on its nitric oxide production and on hypoxic signaling

“…This activity is observed only at very low oxygen concentrations; which is interesting, because both yeast and mammalian cells respond to hypoxia by producing Cco isozymes with altered activities (28)(29)(30). This response is brought about by the differential expression of homologous subunit isoforms whose genes are inversely regulated by O 2 (30)(31)(32)(33)(34)(35). One of these isoforms (either Va or Vb in yeast and IV-1 or IV-2 in mammals) is required for activity of the Cco (33, 36) but is not part of its catalytic center.…”

“…One of these isoforms (either Va or Vb in yeast and IV-1 or IV-2 in mammals) is required for activity of the Cco (33, 36) but is not part of its catalytic center. Under normoxic conditions the aerobic isoform (yeast Va or mammalian IV-1) is expressed, whereas under hypoxic or anoxic conditions the hypoxic isoform (yeast Vb or mammalian IV-2) is expressed (30,34,35,37). Extensive studies with the yeast subunit V isoforms have revealed that the genes (COX5b and COX5a) for these proteins are switched on or off (respectively) at very low O 2 concentrations (0.5-1 M O 2 ) (31) and that the isoforms affect the catalytic properties of Cco (28,29).…”