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Abstract
The normal type I collagen molecule contains two αl (I) chains and one α2 (I) chain. In embryonic chick calvaria, the two chains are synthesized in a 2: 1 ratio, and total polysomes from this tissue contain twice as much mRNA for proal (I) as for proα2 (I).9 To further investigate the mechanism by which synthesis may be coordinated, RNA isolated from various cell fractions of embryonic chick calvaria was translated in a rabbit reticulocyte lysate cell-free system. The procollagen chain products were separated by gel-electrophoresis and densitometrically quantitated from autoradiograms of the gels. Total cellular RNA, total cytoplasmic RNA, and polysomal RNA each directed the synthesis of proal (I) and proa2 (I) in a proportion of 2: 1, whereas no procollagen mRNA activity was found in nonpolysomal cytoplasmic RNA. These results indicate that in the chick bone cells, all compartments contain twice as much proal (I) mRNA as proa2 (I) mRNA, and that virtually all procollagen mRNA in the cytoplasm is poly some-bound. The coordination of procollagen chain synthesis thus presumably occurs at a pretranslational level, through differential rates of formation and/or degradation of the two mRNAs.