The chromosomal arrangement of human alpha-like globin genes: sequence homology and alpha-globin gene deletions.
We report the isolation of a cluster of four alpha-like globin genes from a bacteriophage lambda library of human DNA (Lawn et al., 1978). Analysis of the cloned DNA confirms the linkage arrangement of the two adult alpha-globin genes (alpha 1 and alpha 2) previously derived from genomic blotting experiments (Orkin, 1978) and identifies two additional closely linked alpha-like genes. The nucleotide sequence of a portion of each of these alpha-like genes was determined. One of these sequences is tentatively identified as an embryonic zeta-globin gene (zeta 1) by comparison with structural data derived from purified zeta-globin protein (J. Clegg, personal communication), while the other sequence cannot be matched with any known alpha-like polypeptide sequence (we designate this sequence phi alpha 1). Localization of the four alpha-like sequences on a restriction map of the gene cluster indicates that the genes have the same transcriptional orientation and are arranged in the order 5'-zeta 1-phi alpha 1-alpha 2-alpha 1-3'. Genomic blotting experiments identified a second, nonallelic zeta-like globin gene (phi 2) located 10-12 kb 5' to the cloned zeta-globin gene. Comparison of the locations of restriction sites within alpha 1 and alpha 2 and heteroduplex studies reveal extensive sequence homology within and flanking the two genes. The homologous sequences, which are interrupted by two blocks of nonhomology, span a region of approximately 4 kb. This extensive sequence homology between two genes which are thought to be the products of an ancient duplication event suggests the existence of a mechanism for sequence matching during evolution. One consequence of this arrangement of homologous sequences is the occurrence of two types of deletions in recombinant phage DNA during propagation in E. coli. The locations and sizes of the two types of deletions are indistinguishable from those of the two types of deletions associated with alpha-thalassemia 2 (Embury et al., 1979; Orkin et al., 1979; S. Embury et al., manuscript submitted). This information strongly suggests that the genetic disease is a consequence of unequal crossing over between homologous sequences within and/or surrounding the two adult alpha-globin genes.