Cytochrome oxidase precursor polypeptide

Pool Size of the Precursor Polypeptides of Cytochrome Oxidase... 

The half lives and the pool sizes of the precursor polypeptides were derived from the time courses of labeling shown in Fig. 9. They are summarized in Table II. As can be seen, the precursor pool of the 20,000 Mr subunit of cytochrome oxidase is relatively small compared to the precursor pools of the other cytochrome oxidase subunits. This 20,000 Mr subunit was shown to be produced by the chloramphenicol-sensitive mitochondrial protein synthesis (Fig. 7B). Consequently, soon after the addition of chloramphenicol, the precursor pool of this subunit is exhausted, making a further assembly of the holoenzyme impossible. This explains why, after... 

The data of Table II provide additional information about the subunits. The precursor pool sizes of the three mitochondrially synthesized cytochrome oxidase subunits differ markedly, being 2% for the 20,000 Mr subunit, 5% for the 28,000 Mr subunit, and 10% for the 36,000 Mr subunit. Hence, in the presence of cycloheximide, the assembly of cytochrome oxidase continues only for a limited period of time, i.e., until the smallest cytoplasmic precursor pool is exhausted. The amount of label which appears in the holoenzyme after this period of time decreases as the size of the precursor pool to be passed increases. This fact explains why in the presence of cycloheximide, less radioactivity appeared in the 28,000 Mr and 36,000 Mr subunits than in the 20,000 Mr subunit (Fig. 7B). However, when cycloheximide is removed, the assembly of the holoenzyme recommences, and all of the H-labeled polypeptides can be integrated. In this manner, a higher cycloheximide-resistant radioactive labeling of the 28,000 Mr and the 36,000 Mr subunits of cytochrome oxidase is obtained with N. crassa. ... 

Fig. 11. Coelectrophoresis of the H-labeled 20,000 Mr precursor polypeptide and of the C-labeled cytochrome oxidase. ...

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