MOLECULAR-ORIGIN OF THE MOSAIC SEQUENCE ARRANGEMENTS OF HIGHER PRIMATE ALPHA-GLOBIN DUPLICATION UNITS

The human adult alpha-globin locus consists of three pairs of homology blocks (X, Y, and Z) interspersed with three nonhomology blocks (I, I I, and III), and three alu family repeats, Alu1, Alu2, and Alu3. It ha s been suggested that an ancient primate a-globin-containing unit was ancestral to the X, Y, and Z and the Alu1/Alu2 repeats. However, the e volutionary origin of the three nonhomologous blocks has remained obsc ure. We have now analyzed the sequence organization of the entire adul t alpha-globin locus of gibbon (Hylobates lar). DNA segments homologou s to human block I occur in both duplication units of the gibbon cr-gl obin locus. Detailed interspecies sequence comparisons suggest that no nhomologous blocks I and II, as well as another sequence, IV, were all part of the ancestral alpha-globin-containing unit prior to its tande m duplication, However, sometime thereafter, block I was deleted from the human alpha 1-globin-containing unit, and block I was also deleted from the alpha 2-globin-containing unit in both human and gibbon, The se were probably independent events both mediated by independent illeg itimate recombination processes. Interestingly, the end points of thes e deletions coincide with potential insertion sites of Alu family repe ats, These results suggest that the shaping of DNA segments in eukaryo tic genomes involved the retroposition of repetitive DNA elements in c onjunction with simple DNA recombination processes.