Genes of the R-phycocyanin II locus of marine Synechococcus spp., and comparison of protein-chromophore interactions in phycocyanins differing in bilin composition

CpeF is the bilin lyase that ligates the doubly linked phycoerythrobilin on β-phycoerythrin in the cyanobacterium Fremyella diplosiphon

Phycoerythrin (PE) is a green light-absorbing protein present in the light-harvesting complex of cyanobacteria and red algae. The spectral characteristics of PE are due to its prosthetic groups, or phycoerythrobilins (PEBs), that are covalently attached to the protein chain by specific bilin lyases. Only two PE lyases have been identified and characterized so far, and the other bilin lyases are unknown. Here, using in silico analyses, markerless deletion, biochemical assays with purified and recombinant proteins, and site-directed mutagenesis, we examined the role of a putative lyase-encoding gene, cpeF, in the cyanobacterium Fremyella diplosiphon. Analyzing the phenotype of the cpeF deletion, we found that cpeF is required for proper PE biogenesis, specifically for ligation of the doubly linked PEB to Cys-48/Cys-59 residues of the CpeB subunit of PE. We also show that in a heterologous host, CpeF can attach PEB to Cys-48/Cys-59 of CpeB, but only in the presence of the chaperone-like protein CpeZ. Additionally, we report that CpeF likely ligates the A ring of PEB to Cys-48 prior to the attachment of the D ring to Cys-59. We conclude that CpeF is the bilin lyase responsible for attachment of the doubly ligated PEB to Cys-48/Cys-59 of CpeB and together with other specific bilin lyases contributes to the post-translational modification and assembly of PE into mature light-harvesting complexes.

Biliprotein maturation: the chromophore attachment

Biliproteins are a widespread group of brilliantly coloured photoreceptors characterized by linear tetrapyrrolic chromophores, bilins, which are covalently bound to the apoproteins via relatively stable thioether bonds. Covalent binding stabilizes the chromoproteins and is mandatory for phycobilisome assembly; and, it is also important in biliprotein applications such as fluorescence labelling. Covalent binding has, on the other hand, also considerably hindered biliprotein research because autocatalytic chromophore additions are rare, and information on enzymatic addition by lyases was limited to a single example, an EF-type lyase attaching phycocyanobilin to cysteine-alpha84 of C-phycocyanin. The discovery of new activities for the latter lyases, and of new types of lyases, have reinvigorated research activities in the subject. So far, work has mainly concentrated on cyanobacterial phycobiliproteins. Methodological advances in the process, however, as well as the finding of often large numbers of homologues, opens new possibilities for research on the subsequent assembly/disassembly of the phycobilisome in cyanobacteria and red algae, on the assembly and organization of the cryptophyte light-harvesting system, on applications in basic research such as protein folding, and on the use of phycobiliproteins for labelling.

Diversity and phylogeny of Baltic Sea picocyanobacteria inferred from their ITS and phycobiliprotein operons

Picocyanobacteria of the genus Synechococcus span a range of different colours, from red strains rich in phycoerythrin (PE) to green strains rich in phycocyanin (PC). Here, we show that coexistence of red and green picocyanobacteria in the Baltic Sea is widespread. The diversity and phylogeny of red and green picocyanobacteria was analysed using three different genes: 16S rRNA-ITS, the cpeBA operon of the red PE pigment and the cpcBA operon of the green PC pigment. Sequencing of 209 clones showed that Baltic Sea picocyanobacteria exhibit high levels of microdiversity. The partial nucleotide sequences of the cpcBA and cpeBA operons from the clone libraries of the Baltic Sea revealed two distinct phylogenetic clades: one clade containing mainly sequences from cultured PC-rich picocyanobacteria, while the other contains only sequences from cultivated PE-rich strains. A third clade of phycourobilin (PUB) containing strains of PE-rich Synechococcus spp. did not contain sequences from the Baltic Sea clone libraries. These findings differ from previously published phylogenies based on 16S rRNA gene analysis. Our data suggest that, in terms of their pigmentation, Synechococcus spp. represent three different lineages occupying different ecological niches in the underwater light spectrum. Strains from different lineages can coexist in light environments that overlap with their light absorption spectra.

Two novel phycoerythrin-associated linker proteins in the marine cyanobacterium Synechococcus sp. strain WH8102

The recent availability of the whole genome of Synechococcus sp. strain WH8102 allows us to have a global view of the complex structure of the phycobilisomes of this marine picocyanobacterium. Genomic analyses revealed several new characteristics of these phycobilisomes, consisting of an allophycocyanin core and rods made of one type of phycocyanin and two types of phycoerythrins (I and II). Although the allophycocyanin appears to be similar to that found commonly in freshwater cyanobacteria, the phycocyanin is simpler since it possesses only one complete set of alpha and beta subunits and two rod-core linkers (CpcG1 and CpcG2). It is therefore probably made of a single hexameric disk per rod. In contrast, we have found two novel putative phycoerythrin-associated linker polypeptides that appear to be specific for marine Synechococcus spp. The first one (SYNW2000) is unusually long (548 residues) and apparently results from the fusion of a paralog of MpeC, a phycoerythrin II linker, and of CpeD, a phycoerythrin-I linker. The second one (SYNW1989) has a more classical size (300 residues) and is also an MpeC paralog. A biochemical analysis revealed that, like MpeC, these two novel linkers were both chromophorylated with phycourobilin. Our data suggest that they are both associated (partly or totally) with phycoerythrin II, and we propose to name SYNW2000 and SYNW1989 MpeD and MpeE, respectively. We further show that acclimation of phycobilisomes to high light leads to a dramatic reduction of MpeC, whereas the two novel linkers are not significantly affected. Models for the organization of the rods are proposed.

Crystal structure of R-phycocyanin and possible energy transfer pathways in the phycobilisome

The crystal structure of R-phycocyanin from Polysiphonia urceolata (R-PC-PU) at 2.4 A is reported. The R-PC-PU crystal belongs to space group P4(3)2(1)2 with cell parameters a = 135.1 A, c = 210.0 A, and alpha = beta = gamma = 90 degrees. The structure was determined by molecular replacement. The crystallographic R-factor of the refined model is 0.189 (R(free) = 0.239). Comparison of the microenvironment of chromophore beta 155 in R-PC-PU and in C-PC from Fremyolla diphosiphon (C-PC-FD) reveals that their spectral differences may be caused by their different alpha 28 residues. In the R-PC-PU crystal structure, two (alpha beta)(3) trimers assemble face to face to form a hexamer, and two such hexamers assemble in two novel side-to-side arrangements. Possible models for the energy transfer from phycoerythrin to phycocyanin and from phycocyanin to allophycocyanin are proposed based on several phycobiliprotein crystal structures.

Genetic diversity and phylogeny of toxic cyanobacteria determined by DNA polymorphisms within the phycocyanin locus

Cyanobacteria are a highly diverse group in relation to form, function, and habitat. Current cyanobacterial systematics relies on the observation of minor and plastic morphological characters. Accurate and reliable delineation of toxic and bloom-forming strains of cyanobacteria has not been possible by traditional methods. We have designed general primers to the phycocyanin operon (cpc gene) and developed a PCR which allows the amplification of a region of this gene, including a variable intergenic spacer sequence. Because of the specificity of this PCR for cyanobacterial isolates, the assay is appropriate for the rapid and reliable identification of strains in freshwater samples. Successive restriction endonuclease digestion of this amplification product, with a total of nine enzymes, yielded many identifying DNA profiles specific to the various taxonomic levels of cyanobacteria. The restriction enzyme profiles for MspI, RsaI, and TaqI were conserved for strains within each of the eight genera (40 strains) studied and clearly discriminated among these genera. Intrageneric delineation of strains was revealed by the enzymes AluI, CfoI, and HaeIII for members of the genus Microcystis, while strains of genus Anabaena were differentiated by the digestion patterns provided by AluI, CfoI, and ScrFI. Phenetic and cladistic analyses of the data were used to infer the genetic relatedness and evolution of toxic and bloom-forming cyanobacteria.

Candidate genes for the phycoerythrocyanin alpha subunit lyase. Biochemical analysis of pecE and pecF interposon mutants

The rod substructures of the Anabaena sp. PCC 7120 phycobilisome contain the light harvesting proteins C-phycocyanin and phycoerythrocyanin (PEC). Even at low light intensities, PEC represents no more than 5% of the phycobilisome protein. The beta subunits of both proteins carry thioether-linked phycocyanobilin (PCB) at beta-Cys-82 and beta-Cys-155; however, C-phycocyanin has PCB at alpha-Cys-84 whereas PEC alpha subunit carries phycobiliviolin at this position. The Anabaena sp. PCC 7120 pec operon is made up of five genes. PecB and pecA encode the beta and alpha subunits of PEC, pecC encodes a linker polypeptide associated with PEC in the rod substructure, and pecE and pecF are genes of unknown function that show a high degree of homology to cpcE and cpcF, that encode a C-phycocyanin alpha subunit PCB lyase (Fairchild, C. D., Zhao, J., Zhou, J., Colson, S. E., Bryant, D. A., and Glazer, A. N. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 7017-7021). Insertional mutants in pecE and pecF, and an interposon mutant in which a portion of both pecE and pecF was deleted, were constructed. All three types of mutants grew 1.3 times slower than wild-type under limiting light conditions and showed a 20% reduction in the PCB content of whole cells relative to chlorophyll alpha. Holo-PEC was missing from the phycobilisomes of all three types of mutants and the level of the PEC linker polypeptide was reduced relative to the wild-type. However, approximately 30% of the wild-type level of the PEC beta subunit was present in all of these phycobilisomes. In contrast, the PEC alpha subunit was barely detectable in the pecE and pecF mutants, but was present in the pecEF deletion mutant as a PCB-adduct in a 1:1 ratio with the PEC beta subunit. The identity of this "unnatural" adduct was confirmed by isolation of the subunit and amino-terminal sequencing. These biochemical results support the inference that pecE and pecF encode a PEC alpha subunit phycobiliviolin lyase, and, in conjunction with earlier findings, demonstrate that phycobiliprotein bilin lyases show high selectivity (rather than absolute specificity) for both the bilin and the polypeptide substrate.

The complete amino acid sequence of R-phycocyanin-I alpha and beta subunits from the red alga Porphyridium cruentum. Structural and phylogenetic relationships of the phycocyanins within the phycobiliprotein families

We present here the complete primary structure of R-phycocyanin-I alpha and beta subunits from the red alga Porphyridium cruentum. The alpha chain is composed of 162 amino acid residues (18049 Da, calculated from sequence, including chromophore) and carries a phycocyanobilin pigment covalently linked to Cys84. The beta chain contains 172 amino acids (19344Da, calculated from sequence, including chromophores) and carries a phycocyanobilin pigment covalently linked at Cys82 and a phycoerythrobilin pigment at Cys153. A gamma-N-methyl asparagine residue was also characterised at position beta 72 similar to other phycobiliprotein beta subunits. R-phycocyanin-I from Porphyridium cruentum shares high sequence identity with C-phycocyanins (69-83%), R-phycocyanins (66-70%) and in a less extent with phycoerythrocyanins (57-65%) from various sources. The presented phylogenetic trees are based on a comparison of all phycobiliprotein amino acid sequences known so far and confirm the clear affiliation of the R-phycocyanins in the phycocyanin family. In spite of their particular phycobilin pattern, they do not represent intermediate forms between the phycocyanin and the phycoerythrin family. Phycoerythrocyanin, a phycocyanin-related phycobiliprotein adapted to green light harvesting, is also shown to belong to the phycocyanin family. However, the phycoerythrocyanins diverge from phycocyanins in their different function and it is suggested that they should be assigned to a separate group within the phycocyanin family.