title: Characterization of damage to photosystems I and II in a cyanobacterium lacking detectable iron superoxide dismutase activity.
abstract: The enzyme superoxide dismutase is ubiquitous in aerobic organisms where it plays a major role in alleviating oxygen-radical toxicity. An insertion mutation introduced into the iron superoxide dismutase locus (designated sodB) of the cyanobacterium Synechococcus sp. PCC 7942 created a mutant strain devoid of detectable iron superoxide dismutase activity. Both wild-type and mutant strains exhibited similar photosynthetic activity and viability when grown with 17 mumol.m-2.s-1 illumination in liquid culture supplemented with 3% carbon dioxide. In contrast, the sodB mutant exhibited significantly greater damage to its photosynthetic system than the wild-type strain when grown under increased oxygen tension or with methyl viologen. Although damage occurs at both photosystems I and II, it is primarily localized at photosystem I in the sodB mutant. Growth in 100% molecular oxygen for 24 hr decreased photoacoustically measured energy storage in 3-(3,4-dichlorophenyl)-1,1-dimethylurea and abolished the fluorescence state 2 to state 1 transition in the sodB mutant, indicating interruption of cyclic electron flow around photosystem I. Analysis of the flash-induced absorption transient at 705 nm indicated that the interruption of cyclic electron flow occurred in the return part of the cycle, between the two [4 Fe-4 S] centers of photosystem I, FA and FB, and cytochrome f. Even though the sodB mutant was more sensitive to damage by active oxygen than wild-type cells, both strains were equally sensitive to the photoinhibition of photosystem II caused by exposure to strong light.
authors.affiliation: Department of Plant Biology, Carnegie Institution of Washington, Stanford, CA 94305.
authors.forename: S K
authors.initials: SK
authors.lastname: Herbert
authors.forename: G
authors.initials: G
authors.lastname: Samson
authors.forename: D C
authors.initials: DC
authors.lastname: Fork
authors.forename: D E
authors.initials: DE
authors.lastname: Laudenbach
mesh_terms: D000458:Cyanobacteria / Q000201:enzymology*; D004579:Electron Transport; D005798:Genes, Bacterial; D008027:Light; D016254:Mutagenesis, Insertional; D010100:Oxygen / Q000502:physiology; D010788:Photosynthesis; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism*; D015183:Restriction Mapping; D013050:Spectrometry, Fluorescence; D013482:Superoxide Dismutase / Q000502:physiology*
chemical_list: D045322:Photosynthetic Reaction Center Complex Proteins; D013482:Superoxide Dismutase; D010100:Oxygen