| Abstract: | The normal type I collagen molecule contains two alpha 1 (I) chains and one alpha 2 (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 pro alpha 1 (I) as for pro alpha 2 (I). 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 pro alpha 1 (I) and pro alpha 2 (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 pro alpha 1 (I) mRNA as pro alpha (I) mRNA, and that virtually all procollagen mRNA in the cytoplasm in polysome-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. |
