SHEEP ALPHA-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 alpha-globin genes (I alpha and II alpha), which are expressed at different levels, the upstream gene being the most efficient. In alpha-globin gene trip lication and quadruplication, this trend is confirmed, i.e., the alpha -chain output of the downstream genes progressively decreases. In this study, we have determined the complete sequence of the cDNAs and of b oth the introns in a triple-a haplotype in which each gene could be re cognized for the presence of distinct alleles. The sequence analysis r eveals that the bodies of the three alpha-globin genes are essentially identical (99.9% homology) and moreover indicates that the down-regul ation of additional alpha-globin genes in sheep is not the effect of s equence variation from the Cap to the Poly(A) addition sites. This str iking similarity among alpha-genes is higher than that seen in other m ammals and is probably sustained by particularly efficient mechanisms of gene conversion and cross-over fixation.