Phycourobilin in Trichromatic Phycocyanin from Oceanic Cyanobacteria Is Formed Post-translationally by a

Most cyanobacteria harvest light with large antenna com-plexescalledphycobilisomes.Thediversityoftheirconstitutingphycobiliproteins contributes to optimize the photosyntheticcapacity of these microorganisms. Phycobiliprotein biosynthe-sis, which involves several post-translational modificationsincluding covalent attachment of the linear tetrapyrrole chro-mophores(phycobilins)toapoproteins,beginstobewellunder-stood. However, the biosynthetic pathway to the blue-green-absorbing phycourobilin ( on earth, phototrophic organisms have to cope with large spa-