title: Increased Photochemical Efficiency in Cyanobacteria via an Engineered Sucrose Sink.
abstract: In plants, a limited capacity to utilize or export the end-products of the Calvin-Benson cycle (CB) from photosynthetically active source cells to non-photosynthetic sink cells can result in reduced carbon capture and photosynthetic electron transport (PET), and lowered photochemical efficiency. The down-regulation of photosynthesis caused by reduced capacity to utilize photosynthate has been termed 'sink limitation'. Recently, several cyanobacterial and algal strains engineered to overproduce target metabolites have exhibited increased photochemistry, suggesting that possible source-sink regulatory mechanisms may be involved. We directly examined photochemical properties following induction of a heterologous sucrose 'sink' in the unicellular cyanobacterium Synechococcus elongatus PCC 7942. We show that total photochemistry increases proportionally to the experimentally controlled rate of sucrose export. Importantly, the quantum yield of PSII (ΦII) increases in response to sucrose export while the PET chain becomes more oxidized from less PSI acceptor-side limitation, suggesting increased CB activity and a decrease in sink limitation. Enhanced photosynthetic activity and linear electron flow are detectable within hours of induction of the heterologous sink and are independent of pigmentation alterations or the ionic/osmotic effects of the induction system. These observations provide direct evidence that secretion of heterologous carbon bioproducts can be used as an alternative approach to improve photosynthetic efficiency, presumably by by-passing sink limitation. Our results also suggest that engineered microalgal production strains are valuable alternative models for examining photosynthetic sink limitation because they enable greater control and monitoring of metabolite fluxes relative to plants.
authors.affiliation: MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, MI, USA.|Department of Cell and Molecular Biology, Michigan State University, East Lansing, MI, USA.
authors.forename: Bradley W
authors.initials: BW
authors.lastname: Abramson
authors.affiliation: Department of Pharmacy and Molecular Biotechnology, Heidelberg University, Heidelberg, Germany.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Kachel
authors.affiliation: MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, MI, USA.|Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI, USA.
authors.forename: David M
authors.initials: DM
authors.lastname: Kramer
authors.affiliation: MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, MI, USA ducatdan@msu.edu.|Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI, USA.
authors.forename: Daniel C
authors.initials: DC
authors.lastname: Ducat
mesh_terms: D002244:Carbon / Q000378:metabolism*; D002734:Chlorophyll / Q000378:metabolism; D015536:Down-Regulation; D004579:Electron Transport; D004734:Energy Metabolism; D018506:Gene Expression Regulation, Plant; D008027:Light; D060847:Metabolic Engineering; D010084:Oxidation-Reduction; D010788:Photosynthesis; D045332:Photosystem II Protein Complex / Q000378:metabolism; D013395:Sucrose / Q000378:metabolism*; D046940:Synechococcus / Q000502:physiology* / Q000528:radiation effects
chemical_list: D045332:Photosystem II Protein Complex; D002734:Chlorophyll; D013395:Sucrose; D002244:Carbon
keywords: Calvin–Benson cycle; Cyanobacteria; Energy balance; Photosynthesis; Source–sink; Synechococcus elongatus PCC 7942