Primary sequence, evolution, and repetitive elements of the Gallus gallus (chicken) beta-globin cluster

The DNA sequence of the Gallus gallus (chicken) beta-globin cluster was completed and analyzed. This G + C-rich region is 23.7 kb in length and includes the rho-, beta H-, beta A-, and epsilon-globin genes, the enhancer found between the beta A and epsilon genes, and three upstream DNase I hypersensitive sites. The CpG dinucleotides are nonrandomly distributed, being present at an increased relative frequency near the promoters and upstream hypersensitive sites. The cluster has an unusually low TA dinucleotide frequency. The upstream hypersensitive sites (5'HS1, 5'HS2, and 5'HS3) contain DNA sequence motifs recognized by erythroid transcription factors. However, no significant sequence similarity was found among the upstream hypersensitive sites and the beta A/epsilon enhancer. The G. gallus upstream site sequences were not similar to the upstream sites of the mammalian globin clusters, probably due to the small size of the functional regions and large evolutionary distance between the classes. The avian cluster evolved by gene duplication from an ancestor beta-globin gene, first producing the epsilon and the rho/beta H/beta A ancestor genes, then the rho and the beta H/beta A ancestor genes, and finally the beta H- and beta A-globins. Four probable gene conversions can be documented: beta A to beta H, epsilon to beta H, and rho/epsilon (twice). The cluster shows a massive overrepresentation of a non-LTR retrotransposon, CR1, which accounts for 16% of the DNA. We suggest that the locus is a preferred site for CR1 insertion.