The size and distribution of cytochrome c oxidase subunit IV mRNA between free and membrane-bound polyribosomes.

A length of 990 +/- 80 nucleotides was estimated for the poly(A)-rich mRNA in the higher-molecular-weight precursor of rat liver cytochrome c oxidase subunit IV. Assuming a molecular weight of 19500 for the cytochrome c oxidase subunit IV precursor, a non-coding region of about 350 nucleotides must exist. In experiments on the site of synthesis it is shown that the poly(A)-rich RNA for the higher-molecular-weight precursor of cytochrome c oxidase subunit IV is found in free, loosely and tightly membrane-bound polyribosomes. This finding does not agree with the present picture of free polysomes as the exclusive site of synthesis for mitochondrial proteins.