ИСТИНА

Mitochondrial genome, while being strongly reduced in course ofevolution, still codes for several proteins. The vast majority ofthem are components of the respiratory chain complexes. To pro-duce these proteins, the system of mitochondrial translation ispresented in the organelles, which is in common close to that inbacteria.Translation initiation in bacterial cells is orchestrated by threeprotein factors called IF1, IF2 and IF3. The orthologs of the twolatter proteins are commonly found in mitochondria. However,mitochondrial IF3 could not been identified in several groups oforganisms, including S.cerevisiae, for a long time. Recently wehave shown that baker’s yeast protein Aim23p possesses a func-tion of mtIF3. However, the mitochondrial translation has notbeen stopped in the yeast strain without Aim23p which is surpris-ing taking into account the fact that IF3 is obligatory for thetranslation in bacterial systems. Instead of blocking of mitochon-drial protein synthesis in absence of Aim23p, we observed thetranslational imbalance: the synthesis rate of the complex V sub-units was increased while the synthesis rate of the complex IVsubunits was repressed. Thus, in addition to its general role intranslation initiation, Aim23p might specifically affect the biosyn-thesis of individual mitochondrial-encoded protein species. Ourgenetic experiments have revealed that, indeed, Aim23p is almostindispensable for Cox1p synthesis, and that it affects the transla-tion of COX1 mRNA through its 50-UTR, like classical mito-chondrial translational activators. This is in accordance with ourmeasurements of complex IV activity which is several times lessin yeast lacking AIM23 gene than in the wild-type. Takentogether, our results point on the multiple role of Aim23p inmitochondrial translation: in addition to its function as mito-chondrial IF3, it specifically regulates the amount of complex IVsubunits and its activity.