Cryptomonad biliproteins - An evolutionary perspective

Each cryptomonad strain contains only a single spectroscopic type of biliprotein. These biliproteins are isolated as similar to 50000 kDa alpha alpha'beta(2) complexes which carry one bilin on the alpha and three on the beta subunit. Six different bilins are present on the cryptomonad biliproteins, two of which (phycocyanobilin and phycoerythrobilin) also occur in cyanobacterial and rhodophytan biliproteins, while four are known only in the cryptomonads. The beta subunit is encoded on the chloroplast genome, whereas the a subunits are encoded by a small nuclear multigene family. The beta subunits of all cryptomonad biliproteins, regardless of spectroscopic type, have highly conserved amino acid sequences, which show > 80% identity with those of rhodophytan phycoerythrin beta subunits. In contrast, cyanobacteria and red algal chloroplasts each contain several spectroscopically distinct biliproteins organized into macromolecular complexes (phycobilisomes). The data on biliproteins, as well as several other lines of evidence. indicate that the cryptomonad biliprotein antenna system is 'primitive' and antedates that of the cyanobacteria. It is proposed that the gene encoding the cryptomonad biliprotein beta subunit is the ancestral gene of the gene family encoding cyanobacterial and rhodophytan biliprotein alpha and beta subunits.