Scapharca inaequivalvis tetrameric hemoglobin A and B chains: cDNA sequencing and genomic organization.

A and B globin cDNAs from the tetrameric hemoglobin of the bivalve mollusc Scapharca inaequivalvis were isolated by RT-PCR and sequenced. When compared with the biochemical data, the deduced protein sequences revealed only one amino acid substitution in the B chain. In order to investigate the genomic structure of these invertebrate globin genes, their intronic regions were amplified by PCR. The two genes showed the typical two-intron/three-exon organization found in vertebrates and seemed to reflect the ancestral gene structure, in accordance with the new globin gene evolution theory proposed by Dixon and Pohajadak (Trends Biochem. Sci. 17:486-488, 1992). The alternative hypothesis suggested by Go (Nature 291:90-92, 1981), that the central intron was lost during evolution, is also considered. In contrast to the related clam Anadara trapezia, S. inaequivalvis A and B globin genes were found to be present in multiple copies differing in intron size. In this study we report the complete sequences of the A (1,471 bp) and B (2,221 bp) globin genes, giving a detailed analysis of their intron features.