A hemoglobin from plants homologous to truncated hemoglobins of microorganisms

We have identified a nuclear-encoded Hb from plants (GLB3)that has a centra l domain similar to the "truncated" Hbs of bacteria, protozoa, and algae. T he three-dimensional structure of these Hbs is a 2-on-2 arrangement of alph a -helices, distinct from the 3-on-3 arrangement of the standard globin fol d [Pesce, A., Couture, M Dewilde, S., Guertin, M., Yamauchi, K.,Ascenzi, P. , Moens, L. & Bolognesi, M. (2000) EMBOJ. 19, 2424-2434]. GLB3-like genes a re not found in animals or yeast, but our analysis reveals that they are pr esent in a wide range of Angiosperms and a Bryophyte. Although cyanobacteri a and Chlamydomonas have 2-on-2 Hbs (GLBN), GLB3 is more likely related to GLBO-type 2-on-2 Hbs from bacteria. Consequently, GLB3 is unlikely to have arisen from a horizontal transfer between the chloroplast and nuclear genom es. Arabidopsis thaliana GLB3 protein exhibits unusual concentration-indepe ndent binding Of O-2 and CO. The absorbance spectrum of deoxy-GLB3 is uniqu e; the protein forms a transient six-coordinate structure after reduction a nd deoxygenation, which slowly converts to a five-coordinate structure. In A. thaliana, GLB3 is expressed throughout the plant but responds to none of the treatments that induce plant 3-on-3 Hbs. Our analysis of the sequence, ligand interactions, and expression profile of GLB3 indicates that this pr otein has unique biochemical properties, evolutionary history, and, most li kely, a function distinct from those of other plant Hbs.