Abalone myoglobins evolved from indoleamine dioxygenase: the cDNA-derived amino acid sequence of myoglobin from Nordotis madaka.

The cDNA for the unusual 41 kD myoglobin of the abalone Nordotis madaka was amplified by polymerase chain reaction (PCR), and the cDNA-derived amino acid sequence of 378 residues was determined. As with the myoglobin of the related abalone Sulculus diversicolor (Suzuki and Takagi, J. Mol. Biol. 228, 698-700, 1992), the sequence of Nordotis myoglobin showed no significant homology with any other globins, but showed high homology (35% identity) with vertebrate indoleamine 2,3-dioxygenase, a tryptophan degrading enzyme containing heme. The amino acid sequence homology between Nordotis and Sulculus myoglobins was 87%. These results support our previous idea that the abalone myoglobins evolved from a gene for indoleamine dioxygenase, but not from a globin gene, and therefore all of the hemoglobins ard myoglobins are not homologous. Thus, abalone myoglobins appear to be a typical case of convergent evolution.