The collagens represent an interesting example of a structurally related but genetically distinct family of proteins1. Type I, the most abundant of the vertebrate collagens, comprises two proα1(I) chains and one proα2(I) chain, each containing terminal propeptides and a central domain of 338 (Gly, X, Y) repeats. The structure of the chicken proα2(I) gene shows an intriguing relationship between exon organization and the arrangement of (Gly, X, Y) repeats (see ref. 2 for review). This has led to the suggestion3 that the collagens evolved from a common ancestral unit of 54 base pairs (bp). Here we present the structure of the entire human proα1(I) gene and compare this with the chicken proα2(I). The exon arrangement of the two genes is remarkably similar, although the human proα1(I) is more compact because of the shorter length of its introns. The data strongly support the notion that the type I genes have evolved from an ancestral multi-exon unit, and that once the gene was translated, a strong evolutionary pressure caused it to maintain this elaborate structure.
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