Procollagen biosynthesis by embryonic-chick-bone polysomes. Estimation of the relative numbers of active proalpha1...

Both total polysomes and polysomes of different size classes isolated from embryonic chick cranial bones were allowed to complete their nascent polypeptide chains in a cell-free system containing rabbit reticulocyte post-ribosomal supernatant fraction. In this system, no de novo initiation of polypeptide synthesis occurred. The product was analysed for relative content of proalpha1 and proalpha2, the precursors of the alpha chains of collagen, by dodecylsulphate-acrylamide gel electrophoresis as well as by paper electrophoresis following tryptic digestion. The results showed that the products of polysome protein synthesis contained proalpha1 and proalpha2 in the 2:1 ratio in which the corresponding alpha chains are present in native collagen, and that proalpha1 and proalpha2 synthesising polysomes are of the same size. These findings, in conjunction with results from a previous report (Vuust, J. and Piez, K.A. (1972) J. Biol. Chem. 247, 856-862) suggest that active messenger ribonucleic acids for the proalpha1 and proalpha2 chains, respectively, are present in the cells in a ratio of 2:1, and that the rates of initiation, elongation, termination and release from polysomes are all identical for the two chains.