Characterization of cryptomonad phycoerythrin and phycocyanin

Phycoerythrin, a chromoprotein, from the cryptomonad alga Rhodomonas lens is composed of two pairs of nonidentical polypeptides (α₂β₂). This structure is indicated by a molecular weight of 54,300, calculated from osmotic pressure measurements and by sodium dodecyl sulfate (SDS) gel electrophoresis, which showed bands with molecular weights of 9800 and 17,700 in a 1:1 molar ratio. The s_20,w⁰ of 4.3S is consistent with a protein of this molecular weight. Similar results were obtained with another cryptomonad phycoerythrin and a cryptomonad phycocyanin. Electrophoresis after partial cross-linking by dimethyl suberimidate revealed seven bands for the cryptomonad phycocyanin and six bands for cryptomonad phycoerythrin and confirmed the proposed structure. Spectroscopic studies on α and β subunits of cryptomonad phycocyanin and phycoerythrin were carried out on the separated bands in SDS gels. The individual polypeptides possessed a single absorption band with the following maxima: phycoerythrin (R. lens), α at 565 nm, β at 531 nm; phycocyanin (Chroomonas sp.), α at 644 nm, β at 566 nm. Fluorescence polarization was not constant across the visible absorption band regions of phycoerythrin (R. lens and C. ovata) with higher polarizations located at higher wavelengths, as had also been previously shown for cryptomonad phycocyanin (Chroomonas sp.). Combining the absorption spectra and the polarization results indicates that in each case the β subunit contains sensitizing chromophores and the α subunit fluorescing chromophores. The CD spectra of cryptomonad phycocyanin and both phycoerythrins were similar and were related to the spectra of the individual subunits. In Ouchterlony double-diffusion experiments the cryptomonad phycoerythrins and phycocyanins cross-reacted, with spurring, with phycoerythrin isolated from a red alga. The cryptomonad phycoerythrins were immunochemically very similar to each other and to cryptomonad phycocyanin, with little spurring detected.     

Regeneration of bovine and octopus opsins in situ with natural and artificial retinals

We consider the problem of color regulation in visual pigments for both bovine rhodopsin (lambda max = 500 nm) and octopus rhodopsin (lambda max = 475 nm). Both pigments have 11-cis-retinal (lambda max = 379 nm, in ethanol) as their chromophore. These rhodopsins were bleached in their native membranes, and the opsins were regenerated with natural and artificial chromophores. Both bovine and octopus opsins were regenerated with the 9-cis- and 11-cis-retinal isomers, but the octopus opsin was additionally regenerated with the 13-cis and all-trans isomers. Titration of the octopus opsin with 11-cis-retinal gave an extinction coefficient for octopus rhodopsin of 27,000 +/- 3000 M-1 cm-1 at 475 nm. The absorption maxima of bovine artificial pigments formed by regenerating opsin with the 11-cis dihydro series of chromophores support a color regulation model for bovine rhodopsin in which the chromophore-binding site of the protein has two negative charges: one directly hydrogen bonded to the Schiff base nitrogen and another near carbon-13. Formation of octopus artificial pigments with both all-trans and 11-cis dihydro chromophores leads to a similar model for octopus rhodopsin and metarhodopsin: there are two negative charges in the chromophore-binding site, one directly hydrogen bonded to the Schiff base nitrogen and a second near carbon-13. The interaction of this second charge with the chromophore in octopus rhodopsin is weaker than in bovine, while in metarhodopsin it is as strong as in bovine.     

A Kinetic Model for the Energy Transfer in Phycobilisomes

A kinetic model for the energy transfer in phycobilisome (PBS) rods of Synechococcus 6301 is presented, based on a set of experimental parameters from picosecond studies. It is shown that the enormous complexity of the kinetic system formed by 400-500 chromophores can be greatly simplified by using symmetry arguments. According to the model the transfer along the phycocyanin rods has to be taken into account in both directions, i.e., back and forth along the rods. The corresponding forward rate constants for single step energy transfer between trimeric disks are predicted to be 100-300 ns^-1. The model that best fits the experimental data is an asymmetric random walk along the rods with overall exciton kinetics that is essentially trap-limited. The transfer process from the sensitizing to the fluorescing C-PC phycocyanin chromophores (τ ≈ 10 ps) is localized in the hexamers. The transfer from the innermost phycocyanin trimer to the core is calculated to be in the range 36-44 ns^-1. These parameters lead to calculated overall rod-core transfer times of 102 and 124 ps for rods containing three and four hexamers, respectively. The model calculations confirm the previously suggested hypothesis that the energy transfer from the rods to the core is essentially described by one dominant exponential function. Extension of the model to heterogeneous PBS rods, i.e., PBS containing also phycoerythrin, is straightforward.     

Isolation and characterization of biliprotein aggregates from Acaryochloris marina, a Prochloron-like prokaryote containing mainly chlorophyll d

Phycobiliprotein aggregates were isolated from the prokaryote Acaryochloris marina, containing chlorophyll d as major pigment. In the electron microscope the biliprotein aggregates appear as rod-shaped structures of 26.0×11.3 nm, composed of four ring-shaped subunits 5.8 nm thick and 11.7 nm in diameter. Spectral data indicate that the aggregates contain two types of biliproteins: phycocyanin and an allophycocyanin-type pigment, with very efficient energy transfer from the phycocyanin- to allophycocyanin-type constituent. The chromophore-binding polypeptides of the pigments have apparent molecular masses of 16.2 and 17.4 kDa. They crossreact with antibodies against phycocyanin and allophycocyanin from a red alga.     

Structure, composition, and assembly of paracrystalline phycobiliproteins in Synechocystis sp. strain BO 8402 and of phycobilisomes in the derivative strain BO 9201.

The phycobiliproteins of the unicellular cyanobacterium Synechocystis sp. strain BO 8402 and its derivative strain BO 9201 are compared. The biliproteins of strain BO 8402 are organized in paracrystalline inclusion bodies showing an intense autofluorescence in vivo. These protein-pigment aggregates have been isolated. The highly purified complexes contain phycocyanin with traces of phycoerythrin, corresponding linker polypeptides LR35PC and LR33PE (the latter in a small amount), and a unique colored polypeptide with an M(r) of 55,000, designated L55. Allophycocyanin and the core linker polypeptides are absent. The substructure of the aggregates has been studied by electron microscopy. Repetitive subcomplexes of hexameric stacks of biliproteins form extraordinary long rods associated side by side in a highly condensed arrangement. Evidence that the linker polypeptides LR35PC and LR33PE stabilize the biliprotein hexamers is presented, while the location and function of the colored linker L55 remain uncertain. The derivative strain BO 9201 contains established hemidiscoidal phycobilisomes comprising phycoerythrin, phycocyanin, and allophycocyanin as well as the corresponding linker polypeptides. The core-membrane linker protein (LCM), and two polypeptides with M(r)s of 40,000 and 45,000 which are present in small amounts, exhibit strong cross-reactivity in Western blot (immunoblot) analysis using an antibody directed against the colored LCM of a Nostoc sp. In contrast, strain BO 8402 exhibits no polypeptide with a significant immunological cross-reactivity in Western blot analysis. Physiological and genetic implications of the unusual pigment compositions of both strains are discussed.     

Fluorescence decay kinetics in phycobilisomes isolated from the bluegreen alga Synechococcus 6301

The picosecond fluorescence and energy-transfer kinetics of isolated phycobilisomes from Synechococcus 6301 were studied under low intensity excitation. Different combinations of excitation and emission wavelengths were used in order to monitor selectively the fluorescence of the pigments phycocyanin and allophycocyanin. The relatively long overall energy-transfer time of 120 ps from the phycocyanin rods to the allophycocyanin-core is rationalized in terms of the special structure of the rods being built up of several phycocyanin hexamers in this alga species. The fluorescence lifetime of the terminal chromophores in the core was determined to be 1.8–1.9 ns depending on the excitation wavelength. A fast decay component of 20 ± 10 ps which is most prominent at short emission wavelengths is assigned to arise mainly from energy transfer within the C-phycocyanin-units from ‘sensitizing’ to ‘fluorescing’ chromophores.     

Evolutionary Analysis of Phycobiliproteins: Implications for Their Structural and Functional Relationships

Phycobiliproteins, together with linker polypeptides and various chromophores, are basic building blocks of phycobilisomes, a supramolecular complex with a light-harvesting function in cyanobacteria and red algae. Previous studies suggest that the different types of phycobiliproteins and the linker polypeptides originated from the same ancestor. Here we retrieve the phycobilisome-related genes from the well-annotated and even unfinished cyanobacteria genomes and find that many sites with elevated d _N /d _S ratios in different phycobiliprotein lineages are located in the chromophore-binding domain and the helical hairpin domains (X and Y). Covariation analyses also reveal that these sites are significantly correlated, showing strong evidence of the functional-structural importance of interactions among these residues. The potential selective pressure driving the diversification of phycobiliproteins may be related to the phycobiliprotein-chromophore microenvironment formation and the subunits interaction. Sites and genes identified here would provide targets for further research on the structural-functional role of these residues and energy transfer through the chromophores. [Reviewing Editor: Dr. Rasmus Nielsen]     

A comparison of cryptophytan phycocyanins

The spectroscopically different phycocyanins present in the type strain of Hemiselmis virescens, Millport 64, and in a second strain of this cryptophytan species, Plymouth 157, have been purified and compared. They are similar in native molecular weight and in subunit structure, both containing alpha and beta subunits with molecular weights of approximately 10000 and 19000 respectively. However, they do not have the same chromophore composition. Both subunits of the phycocyanin of Plymouth 157 contain two bilins with absorption maxima at 600 and 660 nm, respectively. Only the beta subunit of Milllport 64 carries these chromophores; its alpha subunit bears another chromophore, with absorption maxima at 368 and 694 nm. The spectroscopic differences between the two native phycocyanins can be entirely accounted for by their differing chromophore compositions. The phycocyanin of Millport 64 is the only biliprotein so far described which contains three chemically different chromophores.     

Under light limiting growth, CpcB lyase null mutants of the Cyanobacterium Synechococcus sp. PCC 7002 are capable of producing pigmented beta phycocyanin but with altered chromophore function

Phycobilisomes are the major light-harvesting complexes for cyanobacteria, and phycocyanin is the primary phycobiliprotein of the phycobilisome rod. Phycocyanobilin chromophores are covalently bonded to the phycocyanin beta subunit (CpcB) by specific lyases which have been recently identified in the cyanobacterium Synechococcus sp. PCC 7002. Surprisingly, we found that mutants missing the CpcB lyases were nevertheless capable of producing pigmented phycocyanin when grown under low-light conditions. Absorbance measurements at 10 K revealed the energy states of the beta phycocyanin chromophores to be slightly shifted, and 77 K steady state fluorescence emission spectroscopy showed that excitation energy transfer involving the targeted chromophores was disrupted. This evidence indicates that the position of the phycocyanobilin chromophore within the binding domain of the phycocyanin beta subunit had been modified. We hypothesize that alternate, less specific lyases are able to add chromophores, with varying effectiveness, to the beta binding sites.     

Diffuse reflectance spectroscopy of fibrous proteins

UV-visible diffuse reflectance (DR) spectra of the fibrous proteins wool and feather keratin, silk fibroin and bovine skin collagen are presented. Natural wool contains much higher levels of visible chromophores across the whole visible range (700-400?nm) than the other proteins and only those above 450?nm are effectively removed by bleaching. Both oxidative and reductive bleaching are inefficient for removing yellow chromophores (450-400?nm absorbers) from wool. The DR spectra of the four UV-absorbing amino acids tryptophan, tyrosine, cystine and phenylalanine were recorded as finely ground powders. In contrast to their UV-visible spectra in aqueous solution where tryptophan and tyrosine are the major UV absorbing species, surprisingly the disulphide chromophore of solid cystine has the strongest UV absorbance measured using the DR remission function F(R)(∞). The DR spectra of unpigmented feather and wool keratin appear to be dominated by cystine absorption near 290?nm, whereas silk fibroin appears similar to tyrosine. Because cystine has a flat reflectance spectrum in the visible region from 700 to 400?nm and the powder therefore appears white, cystine absorption does not contribute to the cream colour of wool despite the high concentration of cystine residues near the cuticle surface. The disulphide absorption of solid L: -cystine in the DR spectrum at 290?nm is significantly red shifted by ~40?nm relative to its wavelength in solution, whereas homocystine and lipoic acid showed smaller red shifts of 20?nm. The large red shift observed for cystine and the large difference in intensity of absorption in its UV-visible and DR spectra may be due to differences in the dihedral angle between the crystalline solid and the solvated molecules in solution.     

Construction and characterization of a phycobiliprotein-less mutant of Synechocystis sp. PCC 6803

A mutant strain of the cyanobacterium Synechocystis PCC 6803, called PAL, (PC^-, apcAB, apcE), lacking phycocyanin, allophycocyanin and the core-membrane linker (Lcm), was constructed. The strain was characterized by absorption and fluorescence spectroscopy. The mutant compensates for the absence of the major PS II antenna by increasing its PS II / PS I ratio. It is stable and grows well albeit more slowly than wild type.     

Photochromic biliproteins from the cyanobacterium Anabaena sp. PCC 7120: lyase activities, chromophore exchange, and photochromism in phytochrome AphA

Photochromic biliproteins can be switched by light between two states, initiated by Z/E photoisomerization of the linear tetrapyrrole chromophore. The cyanobacterium Anabaena sp. PCC 7120 contains three genes coding for such biliproteins, two coding for phytochromes (aphA/B) and one for the alpha subunit of phycoerythrocyanin (pecA). (a) aphA was overexpressed in Escherichia coli with N-terminal His and S tags, and the protein was reconstituted by an optimized protocol with phycocyanobilin (PCB), to yield the photochromic chromoprotein, PCB-AphA, carrying the PCB chromophore. (b) AphA chromophorylation is autocatalytic such as in other phytochromes. (c) AphA chromophorylation is also possible by chromophore transfer from the PCB-carrying biliprotein, phycocyanin (CPC). The autocatalytic transfer is very slow, and it is enhanced more than 100-fold by catalysis of PCB:CpcA lyase and alpha-CPC as donor. (d) Through deletion mutations of aphA, a short sequence IQPHGV [amino acids (aa) 26-31] was found essential for the lyase activity of AphA, indicating an interaction of the N terminus with the chromophore-binding domain around cysteine 259. (e) A motif of at least 23 aa, starting with this sequence and located approximately 250 aa N terminal of the chromophore-binding cysteine, is proposed to relate to the lyase function in plant and most prokaryotic phytochromes. (f) Long-range interactions in AphA are further supported by blue-shifted absorptions (相似文献   

Excitation energy transfer in phycobiliproteins of the cyanobacterium <Emphasis Type="Italic">Acaryochloris marina</Emphasis> investigated by spectral hole burning

The cyanobacterium Acaryochloris marina developed two types of antenna complexes, which contain chlorophyll-d (Chl d) and phycocyanobilin (PCB) as light-harvesting pigment molecules, respectively. The latter membrane-extrinsic complexes are denoted as phycobiliproteins (PBPs). Spectral hole burning was employed to study excitation energy transfer and electron–phonon coupling in PBPs. The data reveal a rich spectral substructure with a total of four low-energy electronic states whose absorption bands peak at 633, 644, 654, and at about 673 nm. The electronic states at ~633 and 644 nm can be tentatively attributed to phycocyanin (PC) and allophycocyanin (APC), respectively. The remaining low-energy electronic states including the terminal emitter at 673 nm may be associated with different isoforms of PC, APC, or the linker protein. Furthermore, the hole burning data reveal a large number of excited state vibrational frequencies, which are characteristic for the chromophore PCB. In summary, the results are in good agreement with the low-energy level structure of PBPs and electron–phonon coupling parameters reported by Gryliuk et al. (BBA 1837:1490–1499, 2014) based on difference fluorescence line-narrowing experiments.     

Phycobilisome structure in the cyanobacteria Mastigocladus laminosus and Anabaena sp. PCC 7120.

Phycobilisomes of the cyanobacteria Mastigocladus laminosus and Anabaena sp. PCC7120 differ from typical tricylindrical, hemidiscoidal phycobilisomes in three respects. Firstly, size comparisons of the core-membrane linker phycobiliproteins (LCM) in different cyanobacteria by SDS/PAGE reveal an apparent molecular mass of 120 kDa for the LCM of M. laminosus and Anabaena sp. PCC7120. This observation suggests that the polypeptides of these species have four linker-repeat domains. Secondly, phycobilisomes of M. laminosus are shown to contain at least three, but most probably four, different rod-core linker polypeptides (LRC). These LRC, which attach the peripheral rods to the core and thereby make phycocyanin/allophycocyanin contacts, have been identified and characterized by N-terminal amino acid sequence analysis. Additionally, electron microscopy of phycobilisomes isolated from M. laminosus and Anabaena sp. PCC7120 reveals similar structures which differ from those of Calothrix sp. PCC7601 with their typical six, peripheral rods. Based upon protein-analytical results and a reinterpretation of the data of [Isono, T. & Katoh, T. (1987) Arch. Biochem. Biophys. 256, 317-324], we discuss structural implications of recent findings on the established hemidiscoidal model for the phycobilisomes of M. laminosus and Anabaena sp. PCC7120. Up to eight peripheral rods are suggested to radiate from a modified core substructure which contains two additional peripheral allophycocyanin hexamer equivalents that serve as the core-proximal discs for two peripheral rods.     

Transposon insertion in genes coding for the biosynthesis of structural components of the Anabaena sp. phycobilisome

Anabaena sp. PCC 7120 mutants defective in phycobiliprotein biosynthesis or phycobilisome assembly were generated by transposon mutagenesis. Four mutants with grossly reduced content of the major phycobiliprotein, phycocyanin, were found to have insertions within the cpcBACDEFG1G2G3G4 operon coding for phycocyanin biosynthesis and assembly. The insertion in mutant B646 separated the promoter from the open reading frames and eliminated production of the phycocyanin (CpcA) and (CpcB) subunits. Insertion in cpcC in mutant B642 eliminated production of the L³⁶ _Rlinker polypeptide required for assembly of phycocyanin into the distal discs of the phycobilisome rod substructures. Mutants B64328 and B64407 had insertions, respectively, in cpcE and cpcF, genes coding for the subunits of the heterodimeric lyase which catalyzes the attachment of phycocyanobilin to the phycocyanin apo- subunit. Mutant SB12, often unable to survive under low light, was found to have an insertion in the apcE gene coding for the large core-membrane linker (L¹²⁸ _CM) that provides the scaffold for assembly of the phycobilisome core. DNA sequencing 3 of apcE revealed genes apcABC, coding for the and subunits of allophycocyanin and for the small core linker L^7.8 _C. Amino acid sequence comparisons showed that the ApcA and ApcB proteins are 37% identical and that each of these polypeptides is highly similar to corresponding polypeptides from the distantly related filamentous strains Calothrix sp. PCC7601 and Mastigocladus laminosus.     

Factors influencing the phycobilisome rod composition of the cyanobacterium Synechococcus sp. PCC 7942: Effects of reduced phycocyanin content, lack of rod-linkers, and over-expression of the rod-terminating linker

Four novel mutants with altered phycobilisomes were constructed in the cyanobacterium Synechococcus 7942 to study factors influencing the rod length and composition. These mutants show (1) reduced phycocyanin content, (2) reduced phycocyanin content combined with loss of the 33 kDa linker, (3) loss of the 30 kDa rod-linker and (4) overexpression of the 9 kDa rod terminating linker. For these mutants we determined the 33 to 27 kDa and 30 to 27 kDa linker ratios in the isolated phycobilisomes and compared these ratios with those in the wild type. The 30 kDa linker can be incorporated into the rods in absence of the 33 kDa linker. The incorporation of the 30 kDa linker is lower in absence of the 33 kDa linker. When the 30 kDa linker is missing, an increase in the level of the 33 kDa linker is seen, indicating that there could be an excess of the 33 kDa linker in the cells. Our results also show that a reduction in the phycocyanin content causes a decrease in the rod length simultaneously with a reduction of the 30/27 linker ratio, without altering the 33/27 ratio. Reduced phycocyanin content and absence of the 33 kDa linker cause a dramatic reduction in the incorporation of the 30 kDa linker into the rods in the mutant B2SMIKM. Over-expression of the 9 kDa linker results in a decreased incorporation of both the 33 and 30 kDa linkers into the rods, the effect being more pronounced for the 30 kDa linker. This result indicates that the level of the 9 kDa linker relative to those of the 33 and the 30 kDa linkers may be an important determinant of the phycobilisome rod length.     

Characterisation of a B800/1020 antenna from the photosynthetic bacteria Ectothiorhodospira halochloris and Ectothiorhodospira abdelmalekii

An antenna fraction designated B800/1020 according to its near-infrared absorption maxima has been isolated from the bacteriochlorophyll b-containing photosynthetic bacterium, Ectothiorhodospira halochloris. It contains five polypeptides (approx. 4.5, 6.0, 6.5, 15.5 and 35 kDa), at least five strongly interacting BChl b chromophores and no carotenoids. Energy is transferred from the chromophores absorbing around 800 nm to the ones absorbing at 1020 nm. The B800/1020 fraction as well as chromatophores, sphaeroplasts or whole bacteria are reversibly transformed with acid to form B800/960 with a pK value of approx. 6.3. Circular dichroism and low-temperature fluorescence data of the ‘low-pH form’ indicate only little structural rearrangement of the chromophores and a retention of the energy transfer.