A hemoglobin with an optical function

Hemoglobins are best known as oxygen transport proteins. Here we describe a hemoglobin from the parasitic nematode Mermis nigrescens (Mn-GLB-E) that h as an optical, light shadowing function. The protein accumulates to high co ncentration as intracellular crystals in the ocellus of mature phototactic adult females while also being expressed at low concentration in other tiss ues. It differs in sequence and expression pattern from Mn-GLB-B, a second Mermis globin. It retains the structure and oxygen-binding and light-absorb ing properties typical of nematode hemoglobins. As such, recruitment to a s hadowing role in the eye appears to have occurred by changes in expression without modification of biochemistry. Both globins are coded by genes inter rupted by two introns at the conserved positions B12.2 and G7.0, which is i n agreement with the 3exon/2intron pattern model of globin gene evolution.