Dodia, RJ; Rich, P; Meuniert, B; KAY, C; (2014) Comparisons of subunit 5A and 5B isozymes of yeast cytochrome c oxidase. Biochemical Journal , 464 (3) pp. 335-342. 10.1042/BJ20140732.

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Abstract

Subunit 5 of Saccharomyces cerevisiae CcO is essential for assembly and has two isoforms, 5A and 5B. 5A is expressed under normoxic conditions while 5B is expressed at very low oxygen tensions. As a consequence, COX5A-deleted strains (Acox5A) have no or only low levels of CcO under normoxic conditions rendering them respiratory-deficient. Previous studies reported that respiratory growth could be restored by combining Acox5A with mutations of ROX1 that encodes a repressor of COX5B expression. In these mutants 5B isozyme expression level was 30-50 % of wild type (5A isozyme) and exhibited a maximum catalytic activity up to 3-fold faster than that of 5A isozyme. 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 isozyme, without the complication of additional effects caused by mutation ofROX1. When produced this way, the isozymes 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 isozyme in the rox1 mutant is not caused simply by exchange of isoforms and must arise from an additional effect that remains to be resolved.

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