Type III collagen is often found in the same tissues as type I collagen, yet the function and nature of the fibrils formed by the two collagens differ markedly1-3. To understand the evolutionary history of the collagen gene family in more detail, we isolated the gene for type III collagen and compared its structure with that of the gene for α2(I) collagen 4-9. This comparison points to a remarkable conservation in the size distribution of the exons coding for the helical part of these two collagen polypeptides: equivalent amino acid segments in the helical domain of each polypeptide are encoded by exons of equal sizes in each gene. This suggests that after the interstitial collagen genes had been duplicated from a common ancestor about 2-5 × 108 years ago10, no recombinations between these exons were tolerated, although the same recombinational phenomena must have played an important part in shaping the structure of the progenitor for these genes. This fixation of the size distribution of the exons which code for the interstitial collagen helical domains is found despite the persistence in these exons of sequence elements that should have favoured recombinational rearrangements, and contrasts with the variations in the pattern of sizes of some exons coding for the amino and carboxyl propeptides of these collagens.
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