Daphnia pulex didomain hemoglobin: Structure and evolution of polymeric hemoglobins and their coding genes

The high-molecular-weight extracellular hemoglobin of Daphnia pulex is comp osed of at least three different di-domain globin chains. The primary struc ture of one of these chains was determined at the protein and cDNA levels. Each globin domain of the polypeptide chain displays the standard structura l characteristics. The first domain is preceded by a 30-residue extension c ontaining an 18-residue unprecedented threonine-rich segment and a 12-resid ue preA segment which is homologous to the preA segments of other nonverteb rate globin chains. Both domains are linked together by a preA' segment, wh ich is homologous to other preA segments and lacks the threonine-rich segme nt. Dimerization of the globin chains by the formation of a disulphide brid ge Linking the unique cysteines near the amino-termini results in a covalen t, vertebrate-like tetradomain structure. The flexible amino-terminal exten sion most Likely facilitates dimerization. The gene coding for this globin chain is interrupted by six small introns. Each domain displays two intrado main introns at the conserved positions B12.2 and G7.0. A precoding intron occurs at position preA(-27.0) and a bridge intron at occurs preA'(-13.2). We propose a crossover event as the most likely mechanism for duplication. Arthropod globin trees reflect the added effects of gene diversification, g ene duplication, and species evolution. The position of monodomain intracel lular globins in the tree suggests that they resemble the ancestral globin more than the derived didomain extracellular globins do.