title: Redox regulation of the antimycin A sensitive pathway of cyclic electron flow around photosystem I in higher plant thylakoids.
abstract: The chloroplast must regulate supply of reducing equivalents and ATP to meet rapid changes in downstream metabolic demands. Cyclic electron flow around photosystem I (CEF) is proposed to balance the ATP/NADPH budget by using reducing equivalents to drive plastoquinone reduction, leading to the generation of proton motive force and subsequent ATP synthesis. While high rates of CEF have been observed in vivo, isolated thylakoids show only very slow rates, suggesting that the activity of a key complex is lost or down-regulated upon isolation. We show that isolation of thylakoids while in the continuous presence of reduced thiol reductant dithiothreitol (DTT), but not oxidized DTT, maintains high CEF activity through an antimycin A sensitive ferredoxin:quinone reductase (FQR). Maintaining low concentrations (~2 mM) of reduced DTT while modulating the concentration of oxidized DTT leads to reversible activation/inactivation of CEF with an apparent midpoint potential of -306 mV (±10 mV) and n=2, consistent with redox modulation of a thiol/disulfide couple and thioredoxin-mediated regulation of the plastoquinone reductase involved in the antimycin A-sensitive pathway, possibly at the level of the PGRL1 protein. Based on proposed differences in regulatory modes, we propose that the FQR and NADPH:plastoquinone oxidoreductase (NDH) pathways for CEF are activated under different conditions and fulfill different roles in chloroplast energy balance.
authors.affiliation: Plant Research Laboratory, 612 Wilson Rd 106, Michigan State University, East Lansing, MI 48824, United States.
authors.forename: Deserah D
authors.initials: DD
authors.lastname: Strand
authors.affiliation: Plant Research Laboratory, 612 Wilson Rd 106, Michigan State University, East Lansing, MI 48824, United States.
authors.forename: Nicholas
authors.initials: N
authors.lastname: Fisher
authors.affiliation: Plant Research Laboratory, 612 Wilson Rd 106, Michigan State University, East Lansing, MI 48824, United States; Cell and Molecular Biology Graduate Program, 2240A Biomedical and Physical Science Building, Michigan State University, East Lansing, MI 48824, United States.
authors.forename: Geoffry A
authors.initials: GA
authors.lastname: Davis
authors.affiliation: Plant Research Laboratory, 612 Wilson Rd 106, Michigan State University, East Lansing, MI 48824, United States; Department of Biochemistry and Molecular Biology, 603 Wilson Rd 212, Michigan State University, East Lansing, MI 48824, United States. Electronic address: kramerd8@msu.edu.
authors.forename: David M
authors.initials: DM
authors.lastname: Kramer
mesh_terms: D000968:Antimycin A / Q000494:pharmacology*; D004583:Electrons; D005288:Ferredoxins / Q000378:metabolism; D010084:Oxidation-Reduction; D045331:Photosystem I Protein Complex / Q000378:metabolism*; D011808:Quinone Reductases / Q000378:metabolism; D020524:Thylakoids / Q000378:metabolism*
chemical_list: D005288:Ferredoxins; D045331:Photosystem I Protein Complex; D000968:Antimycin A; D011808:Quinone Reductases; C073412:sulfide quinone reductase
keywords: Cyclic electron flow; Energy balance; Photosynthesis; Redox regulation; Spinacia oleracea