Supramolecular assemblies of mRNA direct the coordinated synthesis of type I procollagen chains.

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Abstract
Registration of the 3 procollagen .alpha. chains and assembly of the triple-helical procollagen molecules takes place in the rough endoplasmic reticulum, but the exact location and timing of assembly is not known. As part of a study of the mechanism of molecular assembly, intact collagen-producing polyribosomes from embryonic chicken tendon fibroblasts were examined by rotary shadowing and EM. Intact mRNA strands corresponding in length to .apprxeq. 4500 bases and complete procollagen .alpha.(I) chains were observed. The mRNA strands are comprised of 2 mRNA chains. The ribosomes are present in pairs separated along the duplex strand by about 100 nm. The intact polysome is asymmetric; 2 duplex strands join, and large ribosome aggregates appear. These aggregates are dispersed by collagenase digestion, leaving separate duplex strands with ribosome pairs intact. Ribonuclease digestion yields mixtures of monosomes and ribosome aggregates. Sequential ribonuclease and collagenase digestions yield only monosomes. Each ribosome apparently reads 1 mRNA chain, so that each pair is thus translating 2 chains in synchrony. Thus, the complex morphology of the collagen-producing polyribosomes suggests that the organization of a single molecule begins by the organization of the mRNA chains themselves.