Biosynthesis of the ubiquinol‐cytochrome c reductase complex in yeast. Discoordinate synthesis of the 11‐kd subunit in response to increased gene copy number.

In wild‐type yeast cells, steady‐state concentrations of subunits of the ubiquinol‐cytochrome c reductase complex (complex III) and the levels of their translatable mRNAs change coordinately in response to the need for mitochondrial function. Despite this, re‐introduction of the cloned gene for one of the subunits (11 kd) into cells by transformation with a free‐replicating plasmid results in the discoordinate synthesis of this subunit only, without effects on either the synthesis or degradation of the other subunits. The overproduced subunit is associated with the mitochondrial fraction, yet does not interfere with mitochondrial function, as judged by the growth of transformed cells on nonfermentable media. Quantitative analysis of both mRNA and protein levels suggests that both translational controls and elevated turnover of excess protein contribute to a partial compensation for the effects of increased gene dosage in transformed cells. These contain approximately 30 copies of the cloned gene and 15‐30 times the normal level of its mRNA. Nevertheless, synthesis of the 11‐kd protein is only 6‐ to 8‐fold higher than normal, and steady‐state levels are increased only 5‐ to 10‐fold. These findings imply that synthesis of the various subunits of complex III is not tightly coupled and that for the 11‐kd subunit at least, the level of mRNA is likely to be the most important means of regulating protein level. Fine‐tuning may be additionally achieved by control of translation and degradation of excess protein which is not assembled in the complex.