Functional expression of the plant alternative oxidase affects growth of the yeast Schizosaccharomyces pombe

We have investigated the extent to which functional expression of the plant alternative oxidase (from Sauromatum guttatum) in Schizosaccharomyces pomb e affects yeast growth. When cells are cultured on glycerol, the maximum sp ecific growth rate is decreased from 0.13 to 0.11 h(-1) while growth yield is lowered by 20% (from 1.14 x 10(8) to 9.12 x 10(7) cells ml(-1)). Kinetic studies suggest that the effect on growth is mitochondrial in origin. In i solated mitochondria we found that the alternative oxidase actively compete s with the cytochrome pathway for reducing equivalents and contributes up t o 24% to the overall respiratory activity. Metabolic control analysis revea ls that the alternative oxidase exerts a considerable degree of control (22 %) on total electron flux. Furthermore, the negative control exerted by the alternative oxidase on the flux ratio of electrons through the cytochrome and alternative pathways is comparable with the positive control exerted on this flux-ratio by the cytochrome pathway. To our knowledge, this is the f irst paper to report a phenotypic effect because of plant alternative oxida se expression. We suggest that the effect on growth is the result of high e ngagement of the non-protonmotive alternative oxidase in yeast respiration that, consequently, lowers the efficiency of energy conservation and hence growth.