The polyribosomal mRNA--protein complex is a dynamic structure.

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Abstract
The metabolism of mRNA-protein complexes present in polyribosomes of mouse L cells was investigated with the aid of UV light-induced crosslinking of proteins to RNA. A set of at least 7 proteins which are synthesized and become assoicated with mRNA under conditions permitting mRNA synthesis are also synthesized and become associated with mRNA when mRNA synthesis is inhibited with a high dose of actinomycin D. This set includes a protein of Mr [MW] 78,000 which can be crosslinked to poly(A). mRNA-associated proteins apparently exchange in the cytoplasm with a pool of free proteins. A role for mRNA-associated proteins in translation is suggested. The labeling kinetics of mRNA-associated proteins were investigated and were consistent with cytoplasmic addition to mRNA and a half-life of more than 2 h. Addition implications of these findings are discussed.