Light-Induced Energetic Decoupling as a Mechanism for Phycobilisome-Related Energy Dissipation in Red Algae: A Single Molecule Study

BACKGROUND: Photosynthetic organisms have developed multiple protective mechanisms to prevent photodamage in vivo under high-light conditions. Cyanobacteria and red algae use phycobilisomes (PBsomes) as their major light-harvesting antennae complexes. The orange carotenoid protein in some cyanobacteria has been demonstrated to play roles in the photoprotective mechanism. The PBsome-itself-related energy dissipation mechanism is still unclear. METHODOLOGY/PRINCIPAL FINDINGS: Here, single-molecule spectroscopy is applied for the first time on the PBsomes of red alga Porphyridium cruentum, to detect the fluorescence emissions of phycoerythrins (PE) and PBsome core complex simultaneously, and the real-time detection could greatly characterize the fluorescence dynamics of individual PBsomes in response to intense light. CONCLUSIONS/SIGNIFICANCE: Our data revealed that strong green-light can induce the fluorescence decrease of PBsome, as well as the fluorescence increase of PE at the first stage of photobleaching. It strongly indicated an energetic decoupling occurring between PE and its neighbor. The fluorescence of PE was subsequently observed to be decreased, showing that PE was photobleached when energy transfer in the PBsomes was disrupted. In contrast, the energetic decoupling was not observed in either the PBsomes fixed with glutaraldehyde, or the mutant PBsomes lacking B-PE and remaining b-PE. It was concluded that the energetic decoupling of the PBsomes occurs at the specific association between B-PE and b-PE within the PBsome rod. Assuming that the same process occurs also at the much lower physiological light intensities, such a decoupling process is proposed to be a strategy corresponding to PBsomes to prevent photodamage of the photosynthetic reaction centers. Finally, a novel photoprotective role of gamma-subunit-containing PE in red algae was discussed

Regulation of β-phellandrene synthase gene expression, recombinant protein accumulation, and monoterpene hydrocarbons production in Synechocystis transformants

Main conclusionSuccessful application of the photosynthesis-to-fuels approach requires a high product-to-biomass carbon-partitioning ratio. The work points to the limiting amounts of heterologous terpene synthase in cyanobacteria as a potential barrier in the yield of terpene hydrocarbons via photosynthesis. Cyanobacteria like Synechocystis sp. can be exploited as platforms in a photosynthesis-to-fuels process for the generation of terpene hydrocarbons. Successful application of this concept requires maximizing photosynthesis and attaining a high endogenous carbon partitioning toward the desirable product. The work addressed the question of the regulation of β-phellandrene synthase transgene expression in relation to product yield from the terpenoid biosynthetic pathway of cyanobacteria. The choice of strong alternative transcriptional and translational cis-regulatory elements and the choice of the Synechocystis genomic DNA loci for transgene insertion were investigated. Specifically, the β-phellandrene synthase transgene was expressed under the control of the endogenous psbA2 promoter, or under the control of the Ptrc promoter from Escherichia coli with the translation initiation region of highly expressed gene 10 from bacteriophage T7. These heterologous elements allowed for constitutive transgene expression. In addition, the β-phellandrene synthase construct was directed to replace the Synechocystis cpc operon, encoding the peripheral phycocyanin rods of the phycobilisome antenna. Results showed that a 4-fold increase in the cellular content of the β-phellandrene synthase was accompanied by a 22-fold increase in β-phellandrene yield, suggesting limitations in rate and yield by the amount of the transgenic enzyme. The work points to the limiting amount of transgenic terpene synthases as a potential barrier in the heterologous generation of terpene products via the process of photosynthesis