Characterization of the light harvesting proteins of the chromophytic alga, Olisthodiscus luteus (Heterosigma carterae)

Light harvesting complexes in chlorophyll c-containing algae

Besides the so-called 'green lineage' of eukaryotic photosynthetic organisms that include vascular plants, a huge variety of different algal groups exist that also harvest light by means of membrane intrinsic light harvesting proteins (Lhc). The main taxa of these algae are the Cryptophytes, Haptophytes, Dinophytes, Chromeridae and the Heterokonts, the latter including diatoms, brown algae, Xanthophyceae and Eustigmatophyceae amongst others. Despite the similarity in Lhc proteins between vascular plants and these algae, pigmentation is significantly different since no Chl b is bound, but often replaced by Chl c, and a large diversity in carotenoids functioning in light harvesting and/or photoprotection is present. Due to the presence of Chl c in most of the taxa the name 'Chl c-containing organisms' has become common, however, Chl b-less is more precise since some harbour Lhc proteins that only bind one type of Chl, Chl a. In recent years huge progress has been made about the occurrence and function of Lhc in diatoms, so-called fucoxanthin chlorophyll proteins (FCP), where also the first molecular structure became available recently. In addition, especially energy transfer amongst the unusual pigments bound was intensively studied in many of these groups. This review summarises the present knowledge about the molecular structure, the arrangement of the different Lhc in complexes, the excitation energy transfer abilities and the involvement in photoprotection of the different Lhc systems in the so-called Chl c-containing organisms. This article is part of a Special Issue entitled Light harvesting, edited by Dr. Roberta Croce.

Evidence for nucleomorph to host nucleus gene transfer: light-harvesting complex proteins from cryptomonads and chlorarachniophytes

Cryptomonads and chlorarachniophytes acquired photosynthesis independently by engulfing and retaining eukaryotic algal cells. The nucleus of the engulfed cells (known as a nucleomorph) is much reduced and encodes only a handful of the numerous essential plastid proteins normally encoded by the nucleus of chloroplast-containing organisms. In cryptomonads and chlorarachniophytes these proteins are thought to be encoded by genes in the secondary host nucleus. Genes for these proteins were potentially transferred from the nucleomorph (symbiont nucleus) to the secondary host nucleus; nucleus to nucleus intracellular gene transfers. We isolated complementary DNA clones (cDNAs) for chlorophyll-binding proteins from a cryptomonad and a chlorarachniophyte. In each organism these genes reside in the secondary host nuclei, but phylogenetic evidence, and analysis of the targeting mechanisms, suggest the genes were initially in the respective nucleomorphs (symbiont nuclei). Implications for origins of secondary endosymbiotic algae are discussed.

Gene structure of a chlorophyll a/c-binding protein from a brown alga: presence of an intron and phylogenetic implications

A Laminaria saccharina genomic library in the phage EMBL 4 was used to isolate and sequence a full-length gene encoding a fucoxanthin-chlorophyll a/c-binding protein. Contrary to diatom homologues, the coding sequence is interrupted by an intron of about 900 bp which is located in the middle of the transit peptide. The deduced amino acid sequence of the mature protein is very similar to those of related proteins from Macrocystis pyrifera (Laminariales) and, to a lesser extent, to those from diatoms and Chrysophyceae. Seven of the eight putative chlorophyll-binding amino acids determined in green plants are also present. Alignments of different sequences related to the light-harvesting proteins (LHC) demonstrate a structural similarity among the three transmembrane helices and suggest a unique ancestral helix preceded by two beta-turns. The beta-turns are conserved in front of the second helices of the chlorophyll a/c proteins more so than in chlorophyll a/b proteins. Phylogenetic trees generated from sequence data indicate that fucoxanthin-chlorophyll-binding proteins diverged prior to the separation of photosystem I and photosystem II LHC genes of green plants. Among the fucoxanthin-containing algae, LHC I or II families could not be distinguished at this time.

THE CHLOROPHYLL-CAROTENOID PROTEINS OF OXYGENIC PHOTOSYNTHESIS

The chlorophyll-carotenoid binding proteins responsible for absorption and conversion of light energy in oxygen-evolving photosynthetic organisms belong to two extended families: the Chl a binding core complexes common to cyanobacteria and all chloroplasts, and the nuclear-encoded light-harvesting antenna complexes of eukaryotic photosynthesizers (Chl a/b, Chl a/c, and Chl a proteins). There is a general consensus on polypeptide and pigment composition for higher plant pigment proteins. These are reviewed and compared with pigment proteins of chlorophyte, rhodophyte, and chromophyte algae. Major advances have been the determination of the structures of LHCII (major Chl a/b complex of higher plants), cyanobacterial Photosystem I, and the peridinen-Chl a protein of dinoflagellates to atomic resolution. Better isolation methods, improved transformation procedures, and the availability of molecular structure models are starting to provide insights into the pathways of energy transfer and the macromolecular organization of thylakoid membranes.

Molecular study of a light-harvesting apoprotein of Giraudyopsis stellifer (Chrysophyceae)

We have isolated a gene from a library of nuclear DNA for a chlorophyll a/c-binding protein (named Cac for chl a/c by analogy with Cab for chl a/b) of a chromophyte alga, Giraudyopsis stellifer, and sequenced it. The comparison of the deduced amino acid sequence with other chl a/c- and chl a/b-binding protein sequences shows that structural and functional features, i.e. the arrangement 'en X' of the two A and B transmembrane helices and the putative chl a-binding sites, are shared by both Chlorophyta and Chromophyta. Moreover, in contrast to Chlorophyta, a very strong identity is found among Chromophyta in the C helix suggesting a major function associated to this specific region. Nevertheless, the primary structure of the apoprotein does not seem affected by the pigment composition in Chromophyta. As in the few other examples currently known, we confirm that the cac genes are nuclear-encoded and are part of a multigenic family. Northern blots, performed on poly(A)+ mRNA from G: stellifer, give evidence that the cac gene is light-induced at a transcriptional level and that no expression can be observed in the dark.

Isolation of membrane bound light-harvesting-complexes from the dinoflagellates Heterocapsa pygmaea and Prorocentrum minimum

We have isolated Chl a-Chl c-carotenoid binding proteins from the dinoflagellates Prorocentrum minimum and Heterocapsa pygmaea grown under high (500 μmol m(-2) s(-1), HL) and low (35 μmol m(-2) s(-1), LL) light conditions. We compared various isolation procedures of membrane bound light harvesting complexes (LHCs) and assayed the functionality of the solubilized proteins by determining the energy transfer efficiency from the accessory pigments to Chl a by means of fluorescence excitation spectra. The identity of the newly isolated protein-complexes were confirmed by immunological cross-reactions with antibodies raised against the previously described membrane bound Chl a-c proteins (Boczar et al. (1980) FEBS Lett 120: 243-247). Spectroscopic analysis demonstrated the relatedness of these proteins with the recently described Chl-a-c 2-peridinin (ACP) binding protein (Hiller et al. (1993) Photochem Photobiol 57: 125-131; Iglesias Prieto et al. (1993) Phil Trans R Soc London B 338: 381-392). The water-soluble peridinin-Chl a binding-protein (PCP) was not detectable in P. minimum. Two functional forms of ACP with different pigmentation were isolated. A variant of ACP which was isolated from high-light grown cells, that specifically binds increased amounts of diadinoxanthin was compared to the previously described ACPs that bind proportionately more peridinin.

Nucleotide sequence of two cDNAs encoding fucoxanthin chlorophyll a/c proteins in the diatom Odontella sinensis

Two cDNA clones encoding fucoxanthin chlorophyll a/c-binding proteins (FCP) in the diatom Odontella sinensis have been cloned and sequenced. The derived amino acid sequences of both clones are identical, comparison of the corresponding nucleic acids reveals differences only in the third codon position, suggesting a recent gene duplication. The derived proteins are similar to the chlorophyll a/b-binding proteins of higher plants. The presequences for plastid import resemble signal sequences for cotranslational import rather than transit peptides of higher plants. They are very similar to the presequences of FCP proteins in the diatom Phaeodactylum, but different from the presequences of the gamma-subunit of CF0CF1 of Odontella and the peridinin chlorophyll a binding proteins (PCP) of the dinoflagellate Symbiodinium.