Evolution of chick type I procollagen genes

Although the major types of vertebrate collagen have a number of structural properties in common, significant DNA sequence homologies have not been detected between different portions of the helical coding domains within the same gene or between different genes. However, under non-stringen hybridization conditions we found considerable cross-homology within and between alpha 1(I) and alpha 2(I) chick cDNAs in the coding regions for helical sequences. Detailed analyses at the DNA sequence level have led us to propose that the gene for chick pro alpha 2(I) collagen arose from a 9-bp primordial sequence. A consensus sequence for the 9-bp repeat was derived: GGTCCTCCT, which codes for a Gly-Pro-Pro triplet. The primordial ancestor of this 9-bp unit, GGTCCTXCT, apparently underwent duplication and divergence. Each resulting 9-bp sequence was triplicated to form a 27-bp domain, and a condensation event produced a 54-bp domain. This genetic unit then underwent multiple rounds of amplification to form the ancestral gene for the full-length helical section of alpha 2(I). A different 9-bp consensus sequence (GGTCCCCCC) seems to have been the basis of the chick pro alpha 1(I) gene.