title: Phycourobilin in trichromatic phycocyanin from oceanic cyanobacteria is formed post-translationally by a phycoerythrobilin lyase-isomerase.
abstract: Most cyanobacteria harvest light with large antenna complexes called phycobilisomes. The diversity of their constituting phycobiliproteins contributes to optimize the photosynthetic capacity of these microorganisms. Phycobiliprotein biosynthesis, which involves several post-translational modifications including covalent attachment of the linear tetrapyrrole chromophores (phycobilins) to apoproteins, begins to be well understood. However, the biosynthetic pathway to the blue-green-absorbing phycourobilin (lambda(max) approximately 495 nm) remained unknown, although it is the major phycobilin of cyanobacteria living in oceanic areas where blue light penetrates deeply into the water column. We describe a unique trichromatic phycocyanin, R-PC V, extracted from phycobilisomes of Synechococcus sp. strain WH8102. It is evolutionarily remarkable as the only chromoprotein known so far that absorbs the whole wavelength range between 450 and 650 nm. R-PC V carries a phycourobilin chromophore on its alpha-subunit, and this can be considered an extreme case of adaptation to blue-green light. We also discovered the enzyme, RpcG, responsible for its biosynthesis. This monomeric enzyme catalyzes binding of the green-absorbing phycoerythrobilin at cysteine 84 with concomitant isomerization to phycourobilin. This reaction is analogous to formation of the orange-absorbing phycoviolobilin from the red-absorbing phycocyanobilin that is catalyzed by the lyase-isomerase PecE/F in some freshwater cyanobacteria. The fusion protein, RpcG, and the heterodimeric PecE/F are mutually interchangeable in a heterologous expression system in Escherichia coli. The novel R-PC V likely optimizes rod-core energy transfer in phycobilisomes and thereby adaptation of a major phytoplankton group to the blue-green light prevailing in oceanic waters.
authors.affiliation: UPMC-Université Paris 06, Station Biologique, 29682 Roscoff, France, CNRS, UMR 7144, Groupe Plancton Océanique, 29682 Roscoff, France.
authors.forename: Nicolas
authors.initials: N
authors.lastname: Blot
authors.forename: Xian-Jun
authors.initials: XJ
authors.lastname: Wu
authors.forename: Jean-Claude
authors.initials: JC
authors.lastname: Thomas
authors.forename: Juan
authors.initials: J
authors.lastname: Zhang
authors.forename: Laurence
authors.initials: L
authors.lastname: Garczarek
authors.forename: Stephan
authors.initials: S
authors.lastname: Böhm
authors.forename: Jun-Ming
authors.initials: JM
authors.lastname: Tu
authors.forename: Ming
authors.initials: M
authors.lastname: Zhou
authors.forename: Matthias
authors.initials: M
authors.lastname: Plöscher
authors.forename: Lutz
authors.initials: L
authors.lastname: Eichacker
authors.forename: Frédéric
authors.initials: F
authors.lastname: Partensky
authors.forename: Hugo
authors.initials: H
authors.lastname: Scheer
authors.forename: Kai-Hong
authors.initials: KH
authors.lastname: Zhao
mesh_terms: D002843:Chromatin / Q000378:metabolism*; D002942:Circular Dichroism; D000458:Cyanobacteria / Q000235:genetics / Q000378:metabolism*; D019143:Evolution, Molecular; D007535:Isomerases / Q000378:metabolism*; D008190:Lyases / Q000378:metabolism*; D015394:Molecular Structure; D052980:Phycobilins / Q000096:biosynthesis* / Q000737:chemistry / Q000378:metabolism*; D010798:Phycocyanin / Q000378:metabolism*; D010799:Phycoerythrin / Q000378:metabolism*; D010802:Phylogeny; D011499:Protein Processing, Post-Translational; D012623:Seawater / Q000382:microbiology; D013379:Substrate Specificity
chemical_list: D002843:Chromatin; D052980:Phycobilins; C526232:phycoviolobilin; D010798:Phycocyanin; D010799:Phycoerythrin; C011070:phycoerythrobilin; D008190:Lyases; D007535:Isomerases