Determination of the primary sequence of the duck alpha D globin mRNA and comparison of all adult duck and chick globin mRNA sequences.

The nucleotide sequence of the duck alpha D globin mRNA was determined. Its main feature is an exceptionally short 3' non-coding segment of only 46 nucleotides, placed after the coding sequence of 141 codons. The last of the 6 adult globin mRNA of duck and chicken being thus sequenced, a comparison of all their features has become possible. Comparing the duck alpha D mRNA to the related sequence in the chicken, we found greater homology than comparing it to the linked alpha A globin sequence in the same species. Extensive homology can be found for a same globin chain alpha A, alpha D or beta in between different avian species including also the goose and the ostrich; the avian alpha globin chains show a lower degree of sequence conservation in between species than the beta chains. In contrast, within one species the three globin sequences have further diverged. The divergence between the alpha A and alpha D globin within a same species point to individual functional specificity and hence independent evolution and suggest that a mechanism of 'gene conversion' did not operate in between the avian alpha globin genes. Two segments of the amino acid sequence which we named 'A alpha' and 'B alpha' remain homologous in all avian alpha globins; two other regions 'A beta' and 'B beta' are identical in between the beta globins. Segment A is placed at the 5' end of exon II, and segment B at the 3' end of the same exon; some amino acids in those segments are involved in the Heme binding site. Being almost identical in all know mammalian and avian globins of the alpha respectively the beta type, regions A and B seem to represent the best conserved sequences in adult globin mRNA maintained during the divergence of species.