Molecular Biology and Evolution, Vol 16, 1208-1218, Copyright © 1999 by Society for Molecular Biology and Evolution
S Dewilde, ML Van Hauwaert, K Peeters, J Vanfleteren and L Moens
The high-molecular-weight extracellular hemoglobin of Daphnia pulex is
composed of at least three different didomain globin chains. The primary
structure of one of these chains was determined at the protein and cDNA
levels. Each globin domain of the polypeptide chain displays the standard
structural characteristics. The first domain is preceded by a 30-residue
extension containing an 18-residue unprecedented threonine-rich segment and
a 12-residue preA segment which is homologous to the preA segments of other
nonvertebrate globin chains. Both domains are linked together by a preA'
segment, which is homologous to other preA segments and lacks the
threonine-rich segment. Dimerization of the globin chains by the formation
of a disulphide bridge linking the unique cysteines near the amino-termini
results in a covalent, vertebrate-like tetradomain structure. The flexible
amino- terminal extension most likely facilitates dimerization. The gene
coding for this globin chain is interrupted by six small introns. Each
domain displays two intradomain 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, gene duplication, and species evolution.
The position of monodomain intracellular globins in the tree suggests that
they resemble the ancestral globin more than the derived didomain
extracellular globins do.
ORIGINAL ARTICLE
Daphnia pulex didomain hemoglobin: structure and evolution of polymeric hemoglobins and their coding genes
Department of Biochemistry, University of Antwerp, Belgium.
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