Sheep ?-globin gene sequences: Implications for their concerted evolution and for the down-regulation of the 3? genes

In sheep as in man and most other mammals, there are two ?-globin genes (? and ?), which are expressed at different levels, the upstream gene being the most efficient. In ?-globin gene triplication and quadruplication, this trend is confirmed, i.e., the ?-chain output of the downstream genes progressively decreases. In this study, we have determined the complete sequence of the cDNAs and of both the introns in a triple-? haplotype in which each gene could be recognized for the presence of distinct alleles. The sequence analysis reveals that the bodies of the three ?-globin genes are essentially identical (99.9% homology) and moreover indicates that the down-regulation of additional ?-globin genes in sheep is not the effect of sequence variation from the Cap to the Poly(A) addition sites. This striking similarity among ?-genes is higher than that seen in other mammals and is probably sustained by particularly efficient mechanisms of gene conversion and cross-over fixation. © 1995, Springer-Verlag New York Inc. All rights reserved.