Phycocyanobilin synthesis in the unicellular rhodophyte Cyanidium caldarium

Studies on Cyanidium caldarium Phycobiliprotein Pigment Mutants

Phycobiliprotein biosynthesis was investigated in four strains of the unicellular rhodophyte, Cyandium caldarium, with different pigment phenotypes. All strains were incapable of synthesizing phycobiliproteins when grown in the dark. Western blotting experiments showed that dark-grown cells of the wild-type and mutant GGB synthesized the alpha and beta subunit polypeptides of allophyocyanin and phycocyanin after exposure to light for 24 hours, whereas cells of mutant IIIC and GGBY did not. Similarly, light promoted the appearance of allophycocyanin and phycocyanin mRNAs in the wild-type and GGB but not in IIIC and GGBY. However, Southern blots of restricted genomic DNA from the wild type, IIIC, GGBY, and GGB, all hybridized with heterologous phycobiliprotein gene probes and revealed that all four strains contained identical Pst, EcoRI, and Dral restriction fragments containing allophycocyanin and phycocyanin genes. Cells of the wild type and GGB incubated in the dark with the heme precursor. delta-aminolevulinate, synthesized allophycocyanin and phycocyanin apoproteins providing strong evidence for the role of a tetrapyrrole in regulation of phycobiliprotein gene expression. However, cells of IIIC and GGBY incubated in the dark with delta-aminolevulinate did not contain detectable quantities of allophycocyanin or phycocyanin apoproteins. The possible role of a tetrapyrrole in phycobiliprotein gene expression and basis for the genetic lesion in mutants IIIC and GGBY is discussed.

Biosynthesis of phycobilins. Formation of the chromophore of phytochrome, phycocyanin and phycoerythrin

Phycobiliproteins play important roles in photomorphogenesis and photosynthesis. The light-absorbing chromophores of the phycobiliproteins are linear tetrapyrroles (bilins) very similar in structure to the mammalian bile pigments. 5-Aminolaevulinate (5-ALA) is the first committed intermediate in phycobilin synthesis. The biosynthesis of 5-ALA, destined for phycobilins, occurs via the five-carbon pathway, now well established for tetrapyrrole synthesis in plants and distinct from the mammalian pathway. The phycobilins are formed by reduction of biliverdin which results from the synthesis and degradation of haem. This haem is an essential intermediate in the biosynthesis of phycobilins. Phycocyanobilin, the blue-green pigment found in certain algae and cyanobacteria, is formed from biliverdin via phytochromobilin, the chromophore of phytochrome. This leads to the likelihood that phytochromobilin is formed as an end product, or intermediate, in the synthesis of all phycobilins.

Enzymic Transformation of Biliverdin to Phycocyanobilin by Extracts of the Unicellular Red Alga Cyanidium caldarium

Cell-free extracts of the unicellular red alga Cyanidium caldarium catalyze the transformation of biliverdin to a product indistinguishable from phycocyanobilin, the free bilin derived from phycocyanin by methanolysis. Crude cell-free extract requires biliverdin as the only substrate, but after removal of low molecular weight components by gel filtration, the reaction shows an additional requirement for a reduced pyridine nucleotide. Boiled extract is enzymically inactive, activity is not sedimented by high-speed centrifugation, and mesobiliverdin cannot serve as a substrate.Incubation of cell extracts with biliverdin yields two products with very similar spectrophotometric properties in acidic methanol, but which are separable by reverse-phase high pressure liquid chromatography. The same two products are formed by methanolysis of protein-bound phycocyanin chromophore, with the late-eluting one predominating. The two products derived from either phycocyanin methanolysis or cell extract incubation with biliverdin are partially interconvertible and they form the same ethylidine-free isomeric derivative, mesobiliverdin. Their absorption spectra correspond to those of the Z- and E-ethylidine isomers of phycocyanobilin. Based on previous work showing that the major methanolysis product has the E-ethylidine configuration, the other product of methanolysis and enzymic biliverdin transformation is therefore the Z-ethylidine isomer. The time course for formation of the two products during incubation suggests that the early-eluting product is the precursor of the late-eluting one. These results suggest that Z-ethylidine phycocyanobilin is the precursor of the E-ethylidine isomer, and that the latter may be a normal cellular precursor to protein-bound phycocyanin chromophore.

Ferrochelatase activity in the photosynthetic alga Cyanidium caldarium. Development of the enzyme during biosynthesis of photosynthetic pigments

Dark-grown cells of the photosynthetic alga Cyanidium caldarium were shown to contain ferrochelatase activity, which increased markedly when the cells were induced to form pigments by exposure to light. Km values for the crude enzyme preparation were 14.8 microM and 6.5 microM for binding of Co2+ and deuteroporphyrin IX respectively.

Biosynthesis of the chromophore of phycobiliproteins. A study of mesohaem and mesobiliverdin as possible intermediates and further evidence for an algal haem oxygenase

The possible roles of mesohaem and mesobiliverdin as metabolic precursors of phycocyanobilin, the chromophore of phycocyanin, were studied in the unicellular rhodophyte Cyanidium caldarium. Dark-grown cells of this organism, which had been exposed to mesohaem, were either incubated in the dark with 5-aminolaevulinate, which results in excretion of bilins into the suspending medium, or incubated in the light, which results in synthesis of phycocyanin within the cells. By using 14C-labelling, either in the mesohaem or in the 5-aminolaevulinate administered, it was shown that mesohaem is not a precursor of phycocyanobilin in either dark or light systems. However, mesohaem was converted into mesobiliverdin in both systems, a phenomenon that is further evidence for the existence of an algal haem oxygenase. The data also showed that mesobiliverdin is not a precursor of phycocyanobilin. These results suggest that algal bilins are formed via haem degradation to biliverdin in the same way as mammalian bile pigments.

Biosynthesis of phycocyanobilin from exogenous labeled biliverdin in Cyanidium caldarium

Phycocyanin is a major light-harvesting pigment in bluegreen, red, and cryptomonad algae. This pigment is composed of phycocyanobilin chromophores covalently attached to protein. Phycocyanobilin is an open-chain tetrapyrrole structurally close to biliverdin. Biliverdin is formed in animals by oxidative ring-opening of protoheme. Recent evidence indicates that protoheme is a precursor of phycocyanobilin in the unicellular rhodophyte, Cyanidium caldarium. To find out if biliverdin is an intermediate in the conversion of protoheme to phycocyanobilin, [14C]biliverdin was administered along with N-methylmesoporphyrin IX (which blocks endogenous protoheme formation) to growing cells of C. caldarium. To avoid phototoxic effects due to the porphyrin, a mutant strain was used that forms large amounts of both chlorophyll and phycocyanin in the dark. After 12 or 24 h in the dark, cells were harvested and exhaustively extracted to remove free pigments. Next, protoheme was extracted. Phycocyanobilin was then cleaved from the apoprotein by methanolysis. Protoheme and phycocyanobilin were purified by solvent partition, DEAE-Sepharose chromatography, and preparative reverse-phase high-pressure liquid chromatography. Absorption was monitored continuously and fractions were collected for radioactivity determination. Negligible amounts of label appeared in the protoheme-containing fractions. A major portion of label in the eluates of the phycocyanobilin-containing samples coincided with the absorption peak at 22 min due to phycocyanobilin. In a control experiment, [14C]biliverdin was added to the cells after incubation and just before the phycocyanobilin-apoprotein cleavage step. The major peak of label then eluted with the absorption peak at 12 min due to biliverdin, indicating that during the isolation biliverdin is not converted to compounds coeluting with phycocyanobilin. It thus appears that exogenous biliverdin can serve as a precursor to phycocyanobilin in C. caldarium, and that the route of incorporation is direct rather than by degradation and reincorporation of 14C through protoheme.

Bile pigment synthesis in plants. Incorporation of haem into phycocyanobilin and phycobiliproteins in Cyanidium caldarium

A procedure was developed whereby haem was taken up by dark-grown cells of the unicellular rhodophyte Cyanidium caldarium. These cells were subsequently incubated either in the dark with 5-aminolaevulinate, which results in excretion of phycocyanobilin into the suspending medium or incubated in the light, which results in synthesis and accumulation of phycocyanin and chlorophyll a within the cells. Phycocyanobilin was isolated from phycocyanin by cleavage from apoprotein in methanol. Phycocyanobilin prepared from phycocyanin or excreted from cells given 5-aminolaevulinate was methylated and purified by t.l.c. By using 14C labelling either in the haem or in 5-aminolaevulinate administered, haem incorporation into phycocyanobilin was demonstrated in both dark and light systems. Since chlorophyll a synthesized in the light in the presence of labelled haem contained no radioactivity, it was clear that haem was directly incorporated into phycocyanobilin and not first converted into protoporphyrin IX. These results clearly demonstrate phycocyanobilin synthesis via haem and not via magnesium protoporphyrin IX as has also been postulated.

Mechanism of bile-pigment synthesis in algae. 18O incorporation into phycocyanobilin in the unicellular rhodophyte, Cyanidium caldarium

The origin of the lactam oxygen atoms of phycocyanobilin from Cyanidium caldarium was studied using 18O labelling. By inhibiting photosynthesis, a high 18O enrichment was maintained in the gas phase and the resulting incorporation of label showed that the lactam oxygen atoms were derived from two oxygen molecules. Slow exchange of these oxygen atoms with water was demonstrated directly by using H218O.

The native forms of the phycobilin chromophores of algal biliproteins. A clarification

Pigments released from phycoerythrins and phycocyanins by treatment with hot methanol are currently regarded as equivalent to the native chromophores phycoerythrobilin and phycocyanobilin. However, evidence presented here confirms the original view of O'Carra & O'hEocha [(1966 Phytochemistry 5, 993-997] that these methanol-released pigments are artefacts differing in their chromophoric conjugated systems from the native protein-bound prosthetic groups. By contrast, the native spectral properties are retained in pigments released by careful acid treatment of the biliproteins and these acid-released phycobilins, rather than the methanol-released pigments, are therefore regarded as the protein-free forms of the native chromophores. The conclusion reached by Chapman, Cole & Siegelman [(1968) J. Am. Chem. Soc. 89, 3643-3645], that all the algal biliproteins contain only phycoerythrobilin and phycocyanobilin, is shown to be incorrect. The identification of a urobilinoid chromophore, phycourobilin, accompanying phycoerythrobilin in B- and R- phycoerythrins is confirmed and supported by more extensive evidence. The cryptomonad phycocyanins are shown to contain a phycobilin chromophore accompanying phycocyanobilin. This further phycobilin has the spectral properties of the class of bilins known as violins and the provisional name "cryptoviolin" is proposed pending elucidation of its structure.