The "SUN" family:: UTH1, an ageing gene, is also involved in the regulation of mitochondria biogenesis in Saccharomyces cerevisiae

Since it was shown in previous work that NCA3 (one of the four genes of the SUN family) is involved in mitochondrial protein synthesis regulation, the effect of the other members-of this gene family was tested. UTH1 (but not SUN4 or SIM1) was also shown to interfere with mitochondria biogenesis. In Delta uth1 cells, cytochromes aa(3), c, and b were lowered by 25 and 15%, respectively. In the double-null mutant Delta uth1 Delta nca3, only cytochrome aa(3) was lowered by 50% relative to the wild type. However, the ratio of cellular respiration to cytochrome oxidase was greatly enhanced in the double-null mutant. Measurements on whole lysed cells showed that another mitochondrial enzyme, citrate synthase, was also lowered in Delta uth1 and Delta uth1 Delta nca3 whereas hexokinase was not. Electron micrographs showed no difference in global mitochondria content in Delta uth1 Delta nca3, but mitochondria appeared less dense to electrons compared to the wild type. Cardiolipin and mtDNA were equivalent in parental and mutant strains. Measurements an isolated mitochondria showed that the cyt aa(3)/cyt b ratio was also lowered in Delta uth1 Delta nca3, but the control exerted by the oxidase on the respiratory flux was higher. The activity of other mitochondrial complexes versus oxidase was equivalent in mutants compared to the wild type. These results suggest that the protein equipment could be lowered in mitochondria from strains inactivated for UTH1. (C) 2000 Academic Press.