CRYPTOMONAD BILIPROTEINS - AN EVOLUTIONARY PERSPECTIVE

Each cryptomonad strain contains only a single spectroscopic type of b iliprotein. These biliproteins are isolated as similar to 50000 kDa al pha alpha'beta(2) complexes which carry one bilin on the alpha and thr ee on the beta subunit. Six different bilins are present on the crypto monad biliproteins, two of which (phycocyanobilin and phycoerythrobili n) also occur in cyanobacterial and rhodophytan biliproteins, while fo ur 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 bilipro teins, regardless of spectroscopic type, have highly conserved amino a cid sequences, which show > 80% identity with those of rhodophytan phy coerythrin beta subunits. In contrast, cyanobacteria and red algal chl oroplasts each contain several spectroscopically distinct biliproteins organized into macromolecular complexes (phycobilisomes). The data on biliproteins, as well as several other lines of evidence. indicate th at the cryptomonad biliprotein antenna system is 'primitive' and anted ates 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.