title: An alternate photosynthetic electron donor system for PSI supports light dependent nitrogen fixation in a non-heterocystous cyanobacterium, Plectonema boryanum.
abstract: Plectonema boryanum exhibits temporal separation of photosynthesis and nitrogen fixation under diazotrophic conditions. During nitrogen fixation, the photosynthetic electron transport chain becomes impaired, which leads to the uncoupling of the PSII and PSI activities. A 30-40% increase in PSI activity and continuous generation of ATP through light-dependent processes seem to support the nitrogen fixation. The use of an artificial electron carrier that shuttles electrons between the plastoquinone pool and plastocyanin, bypassing cytochrome b/f complex, enhanced the photosynthetic electron transport activity five to six fold during nitrogen fixation. Measuring of full photosynthetic electron transport activity using methyl voilogen as a terminal acceptor revealed that the photosynthetic electron transport components beyond plastocyanin might be functional. Further, glycolate can act as a source of electrons for PSI for the nitrogen fixing cells, which have residual PSII activity. Under conditions when PSI becomes largely independent of PSII and glycolate provides electrons for PSI activity, the light-dependent nitrogen fixation also was stimulated by glycolate. These results suggest that during nitrogen fixation, when the photosynthetic electron transport from PSII is inhibited at the level of cytochrome b/f complex, an alternate electron donor system for PSI may be required for the cells to carry out light dependent nitrogen fixation.
authors.affiliation: Molecular Biology & Agriculture Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India. hsmisra@magnum.barc.ernet.in
authors.forename: Hari S
authors.initials: HS
authors.lastname: Misra
authors.forename: Nivedita P
authors.initials: NP
authors.lastname: Khairnar
authors.forename: Suresh K
authors.initials: SK
authors.lastname: Mahajan
mesh_terms: D000458:Cyanobacteria / Q000187:drug effects / Q000378:metabolism* / Q000528:radiation effects; D003573:Cytochrome b Group / Q000378:metabolism; D045346:Cytochrome b6f Complex; D004579:Electron Transport; D006016:Glycolates / Q000494:pharmacology; D008027:Light; D009586:Nitrogen Fixation / Q000187:drug effects; D010101:Oxygen Consumption / Q000187:drug effects; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism* / Q000528:radiation effects*; D010970:Plastocyanin / Q000378:metabolism; D010971:Plastoquinone / Q000378:metabolism; D013772:Tetramethylphenylenediamine / Q000494:pharmacology
chemical_list: D003573:Cytochrome b Group; D006016:Glycolates; D045322:Photosynthetic Reaction Center Complex Proteins; C031149:glycolic acid; D010970:Plastocyanin; D045346:Cytochrome b6f Complex; D010971:Plastoquinone; D013772:Tetramethylphenylenediamine