Collagen Polysomes

In the work described above, the collagen messenger activity was detected in total RNA prepared from a mixture of polysomes covering a wide range of sizes. The majority of the RNA preparation is ribosomal, and it is likely that collagen mRNA constitutes only a small proportion of the remainder, but appears to be detectable. 

In the case of type I, collagen biosynthesis (Fig. 12.20a-h) involves first the synthesis of pro-a - and pro-a -precursor chains. The N-ter-minus of these precursors contains up to 25% of extended a - and -chains (a). Immediately after the chains are released from polysomes, hydroxylation of the proline and lysine residues occurs (cf. reactions under 12.20). 

Fig. 12.20. Collagen biosynthesis (according to Bom-stein, 1974). a Polysome, b hydroxylation, c chain straightening, d disulfide bond formation, e cell membrane, f membrane crossing, g a limited hydrolysis to tropocollagen, h collagen fiber formation, cross-linking...

While there is evidence that some modification reactions take place while the growing polypeptide chain is still attached to the polysomes, most modifications occur after release of the chain from the ribosome. Examples of processing of chains still on the ribosome include formation of hydroxy lysine and hydroxyproline in collagen elaboration. 

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