The seal myoglobin gene: an unusually long globin gene

The tetrameric haemoglobins of vertebrates are encoded by alpha- and beta-globin gene families which arose during evolution by a succession of gene duplications, commencing with an alpha beta globin gene duplication which occurred about 500 Myr ago, early in the evolution of the vertebrates. All functional alpha- and beta- globin genes analysed so far share common features, including coding sequence homologies and the presence of two introns within the coding sequence at locations which correlate with interdomain boundaries within globin polypeptides. Here we describe the isolation and characterization of an additional diverged member of the globin gene family, the seal myoglobin gene. We show that monomeric myoglobin, which diverged from haemoglobin about 600-800 Myr ago before the appearance of tetrameric haemoglobins, is also specified by a gene containing two introns at positions precisely homologous to haemoglobin introns. Unlike vertebrate haemoglobin genes, however, the noncoding regions of the seal myoglobin gene are remarkably long.