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-greenabsorbing phycourobilin ( max ϳ 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 ␣-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.To perform photosynthesis, the main energetic basis for life on earth, phototrophic organisms have to cope with large spatial and temporal variations of light conditions. A major evolutionary step in meeting this challenge was the development of light-harvesting complexes, the most variable part of the photosynthetic apparatus (1). By binding a large number of chromophores, these antennas can considerably enhance the photon absorption capacity of reaction centers that are responsible for the conversion of solar energy into chemical energy. Pigmented proteins associated with light-harvesting complexes also fill (at least partially) the large gap between the absorption bands of reaction center chlorophylls (e.g. ϳ440 and 680 nm for chlorophyll a found in most oxygenic organisms). Antennas also transport the excitons with minimal loss and transduce high energy excitons into the low energy ones required by the reaction centers (1, 2). They do not only vary among the different organisms but also with time within individual organisms, thereby providing the flexibility needed by the photosynthetic apparatus to work efficiently under varying ambient conditions. Cyanobacteria, which contribute a substantial fraction of global photosynthesis (...

Phycoerythrin-specific bilin lyase–isomerase controls blue-green chromatic acclimation in marineSynechococcus

“…Spectral analyses of purified wild-type HT-MpeA from MpeZcontaining E. coli cells revealed absorbance and fluorescence emission maxima at 495 and 510 nm, respectively ( Fig. 2A), which matched the spectral properties of PUB attached to protein (8,18,19). HT-MpeA was detectable on protein gels and contained an attached bilin (Fig.…”

“…PEs may bind two different types of chromophores, green light (GL)-absorbing phycoerythrobilin (PEB) and blue light (BL)-absorbing phycourobilin (PUB). These chromophores are ligated to PE by PEB lyases (6,7) or PEB-lyase-isomerases, which both attach the chromophore and isomerize it to PUB (8). No PEspecific PEB-lyase-isomerase has been described to date.…”

Distinct phytochrome actions in nonvascular plants revealed by targeted inactivation of phytobilin biosynthesis

“…1A, the PebA-like protein initially converted BV to produce 15,16-DHBV that was subsequently converted to a new product. The absorption spectrum of the new product was identical to that of phycourobilin (PUB) with maximum absorbance at 492 nm (18,19). In agreement with this interpretation, the molecular mass of the new product determined from mass spectroscopy was 4 Da larger than BV (Fig.…”

Orange fluorescent proteins constructed from cyanobacteriochromes chromophorylated with phycoerythrobilin

“…Here, the molar ratio between PEB and PUB is close to 1 : 1, in view of the fact that in native biliproteins the molar extinction coefficient of PUB is about 1.5-fold bigger than that of PEB. 28,29,34 The autocatalytic isomerization of PEB to PUB has been reported before in one GAF-domain of CBCR from Nostoc punctiforme ATCC 29133. 16 In a R-phycocyanin 34 and phycoerythrin, 35 the isomerization is catalyzed by a lyase.…”

“…28,29,34 The autocatalytic isomerization of PEB to PUB has been reported before in one GAF-domain of CBCR from Nostoc punctiforme ATCC 29133. 16 In a R-phycocyanin 34 and phycoerythrin, 35 the isomerization is catalyzed by a lyase. The reaction is similar to the isomerization of PCB to PVB that is lyase-catalyzed in the α-subunit of phycoerythrocyanin, 36,37 and autocatalytic in several CBCRs that contain GAF domains carrying a conserved DXCF motif.…”