title: Comparisons of subunit 5A and 5B isoenzymes of yeast cytochrome c oxidase.
abstract: Subunit 5 of Saccharomyces cerevisiae cytochrome c oxidase (CcO) is essential for assembly and has two isoforms, 5A and 5B. 5A is expressed under normoxic conditions, whereas 5B is expressed at very low oxygen tensions. As a consequence, COX5A-deleted strains (Δcox5A) have no or only low levels of CcO under normoxic conditions rendering them respiratory deficient. Previous studies have reported that respiratory growth could be restored by combining Δcox5A with mutations of ROX1 that encodes a repressor of COX5B expression. In these mutants, 5B isoenzyme expression level was 30-50% of wild-type (5A isoenzyme) and exhibited a maximum catalytic activity up to 3-fold faster than that of 5A isoenzyme. To investigate the origin of this effect, we constructed a mutant strain in which COX5B replaced COX5A downstream of the COX5A promoter. This strain expressed wild-type levels of the 5B isoenzyme, without the complication of additional effects caused by mutation of ROX1. When produced this way, the isoenzymes displayed no significant differences in their maximum catalytic activities or in their affinities for oxygen or cytochrome c. Hence the elevated activity of the 5B isoenzyme in the rox1 mutant is not caused simply by exchange of isoforms and must arise from an additional effect that remains to be resolved.
authors.affiliation: *Institute of Structural and Molecular Biology, University College London, Gower Street, London WC1E 6BT, U.K.
authors.forename: Raksha
authors.initials: R
authors.lastname: Dodia
authors.affiliation: †Centre de Génétique Moléculaire du CNRS, UPR 3404, avenue de la Terrasse, Gif-sur-Yvette Cedex 91198, France.
authors.forename: Brigitte
authors.initials: B
authors.lastname: Meunier
authors.affiliation: *Institute of Structural and Molecular Biology, University College London, Gower Street, London WC1E 6BT, U.K.
authors.forename: Christopher W M
authors.initials: CW
authors.lastname: Kay
authors.affiliation: *Institute of Structural and Molecular Biology, University College London, Gower Street, London WC1E 6BT, U.K.
authors.forename: Peter R
authors.initials: PR
authors.lastname: Rich
mesh_terms: D000818:Animals; D003576:Electron Transport Complex IV / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D015971:Gene Expression Regulation, Enzymologic; D006736:Horses; D007527:Isoenzymes; D007700:Kinetics; D010100:Oxygen / Q000378:metabolism; D021122:Protein Subunits; D012441:Saccharomyces cerevisiae / Q000201:enzymology* / Q000235:genetics / Q000254:growth & development; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
chemical_list: D007527:Isoenzymes; D021122:Protein Subunits; D029701:Saccharomyces cerevisiae Proteins; C530066:Cox5a protein, S cerevisiae; C530067:Cox5b protein, S cerevisiae; D003576:Electron Transport Complex IV; D010100:Oxygen