Exchange of a single amino acid residue in the cryptophyte phycobiliprotein lyase GtCPES expands its substrate specificity

Cryptophytes are among the few eukaryotes employing phycobiliproteins (PBP) for light harvesting during oxygenic photosynthesis. In contrast to cyanobacterial PBP that are organized in membrane-associated phycobilisomes, those from cryptophytes are soluble within the chloroplast thylakoid lumen. Their light-harvesting capacity is due to covalent linkage of several open-chain tetrapyrrole chromophores (phycobilins). Guillardia theta utilizes the PBP phycoerythrin 545 with 15,16-dihydrobiliverdin (DHBV) in addition to phycoerythrobilin (PEB) as chromophores. The assembly of PBPs in cryptophytes involves the action of PBP-lyases as shown for cyanobacterial PBP. PBP-lyases facilitate the attachment of the chromophore in the right configuration and stereochemistry. Here we present the functional characterization of the eukaryotic S-type PBP lyase GtCPES. We show GtCPES-mediated transfer and covalent attachment of PEB to the conserved Cys82 of the acceptor PBP β-subunit (PmCpeB) of Prochlorococcus marinus MED4. On the basis of the previously solved crystal structure, the GtCPES binding pocket was investigated using site-directed mutagenesis. Thereby, amino acid residues involved in phycobilin binding and transfer were identified. Interestingly, exchange of a single amino acid residue Met67 to Ala extended the substrate specificity to phycocyanobilin (PCB), most likely by enlarging the substrate-binding pocket. Variant GtCPES_M67A binds both PEB and PCB forming a stable, colored complex in vitro and produced in Escherichia coli. GtCPES_M67A is able to mediate PCB transfer to Cys82 of PmCpeB. Based on these findings, we postulate that this single amino acid residue has a crucial role for bilin binding specificity of S-type phycoerythrin lyases but additional factors regulate handover to the target protein.

Energy transfer pathways in the CAC light-harvesting complex of Rhodomonas salina

Photosynthetic organisms had to evolve diverse mechanisms of light-harvesting to supply photosynthetic apparatus with enough energy. Cryptophytes represent one of the groups of photosynthetic organisms combining external and internal antenna systems. They contain one type of immobile phycobiliprotein located at the lumenal side of the thylakoid membrane, together with membrane-bound chlorophyll a/c antenna (CAC). Here we employ femtosecond transient absorption spectroscopy to study energy transfer pathways in the CAC proteins of cryptophyte Rhodomonas salina. The major CAC carotenoid, alloxanthin, is a cryptophyte-specific carotenoid, and it is the only naturally-occurring carotenoid with two triple bonds in its structure. In order to explore the energy transfer pathways within the CAC complex, three excitation wavelengths (505, 590, and 640 nm) were chosen to excite pigments in the CAC antenna. The excitation of Chl c at either 590 or 640 nm proves efficient energy transfer between Chl c and Chl a. The excitation of alloxanthin at 505 nm shows an active pathway from the S₂ state with efficiency around 50%, feeding both Chl a and Chl c with approximately 1:1 branching ratio, yet, the S₁-route is rather inefficient. The 57 ps energy transfer time to Chl a gives ~25% efficiency of the S₁ channel. The low efficiency of the S₁ route renders the overall carotenoid-Chl energy transfer efficiency low, pointing to the regulatory role of alloxanthin in the CAC antenna.

Preferential Plastid Retention by the Acquired Phototroph Mesodinium chamaeleon

The ciliate genus Mesodinium contains species that rely to varying degrees on photosynthetic machinery stolen from cryptophyte algal prey. Prey specificity appears to scales inversely with this reliance: The predominantly phototrophic M. major/rubrum species complex exhibits high prey specificity, while the heterotrophic lineages M. pulex and pupula are generalists. Here, we test the hypothesis that the recently described mixotroph M. chamaeleon, which is phylogenetically intermediate between M. major/rubrum and M. pulex/pupula, exhibits intermediate prey preferences. Using a series of feeding and starvation experiments, we demonstrate that M. chamaeleon grazes and retains plastids at rates which often exceed those observed in M. rubrum, and retains plastids from at least five genera of cryptophyte algae. Despite this relative generality, M. chamaeleon exhibits distinct prey preferences, with higher plastid retention, mixotrophic growth rates and efficiencies, and starvation tolerance when offered Storeatula major, a cryptophyte that M. rubrum does not appear to ingest. These results suggest that niche partitioning between the two acquired phototrophs may be mediated by prey identity. M. chamaeleon appears to represent an intermediate step in the transition to strict reliance on acquired phototrophy, indicating that prey specificity may evolve alongside degree of phototrophy.

Size-selective toxicity effects of the antimicrobial tylosin on estuarine phytoplankton communities

The purpose of this study was to determine the lethal and sublethal effects of the antimicrobial tylosin on natural estuarine phytoplankton communities. Bioassays were used in experimental treatments with final concentrations of 5 to 1000 μg tylosin l(-1). Maximum percent inhibition ranged from 57 to 85% at concentrations of 200-400 μg tylosin l(-1). Half maximum inhibition concentrations of tylosin were ca. 5x lower for small phytoplankton (<20 μm) relative to larger phytoplankton (>20 μm) and suggests that small phytoplankton are more sensitive to tylosin exposure. Sublethal effects occurred at concentrations as low as 5 μg tylosin l(-1). Environmental concentrations of tylosin (e.g., 0.2-3 μg l(-1)) may have a significant sublethal effect that alters the size structure and composition of phytoplankton communities. The results of this study highlight the potential importance of cell size on toxicity responses of estuarine phytoplankton.