ZfN: Search Results

1	Author	Wim Vermaas, Jeroen Charité, Gaozhong Shen	Requires cookie*
	Title	Q a Binding to D2 Contributes to the Functional and Structural Integrity of Photosystem II
	Abstract	Two D 2 mutants were created with a site-directed mutation near the presumable binding site of Q a. In one of the mutants, in which Trp-253, the aromatic residue potentially involved in facilitating electron transport from pheophytin to Q A and/or in binding of Q A, had been replaced by Leu, PS II was undetectable in thylakoids. This mutant is an obligate photohetero-troph. In another mutant the Gly-215 residue, located next to the His residue that is pro posed to bind Q a and Fe2+, was mutated to Trp. This mutation leads to a rapid inactivation of oxygen evolution capacity in the light, and to a virtual elimination of the potential to grow photoautotrophically, but does not greatly affect the number of photosystem II reaction cen ters on a chlorophyll basis. We propose that proper binding of Q A to the photosystem II reac tion center complex is a prerequisite for stability of the photosystem II complex. Impairment of Q a binding leads to rapid inactivation of photosystem II, which may be followed by a struc tural disintegration of the complex.
	Reference	Z. Naturforsch. 45c, 359—365 (1990); received November 3 1989
	Published	1990
	Keywords	Photoinhibition, Plastoquinone, Photosynthesis, Site-Directed Mutagenesis, Cyanobacteria
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	TEI-XML for	default:Reihe_C/45/ZNC-1990-45c-0359.pdf
	Identifier	ZNC-1990-45c-0359
	Volume	45

2	Author	MarcelA K Jansen, ChristoM. Alan, Yoseph Shaaltiel, Jonathan Gressel	Requires cookie*
	Title	Mode of Evolved Photooxidant Resistance to Herbicides and Xenobiotics
	Abstract	A few species have evolved resistance to paraquat after repeated selection. As paraquat still inhibited N A D P reduction, we hypothesized that resistance might be due to (a) detoxification o f the paraquat-generated active oxygen species and (b) that resistant plants would have some cross resistance to other xenobiotic oxidants as well as to photoinhibition, which we subse quently demonstrated. The levels o f plastid isozymes o f the oxygen detoxification pathway: (CuZn) superoxide dismutase, ascorbate peroxidase and glutathione reductase were genetical ly higher in the resistant than in the sensitive biotype o f C onyza bonariensis through the F2 generation. Resistance was suppressed by chelators o f copper and/or zinc. Intact chloroplasts from resistant plants had less membrane damage with and without paraquat, than those from sensitive plants. Resistant C onyza plants recover from paraquat inhibition o f photosynthesis in 3 -4 h in high light, whereas sensitive plants died. Both resistant and sensitive plants recov ered from paraquat in 3 -4 h in low light intensities. Paraquat-resistant Conyza plants were cross-tolerant to S 0 2, atrazine, acifluorfen and to photoinhibition. Drought-tolerant maize inbreds were cross-tolerant to paraquat, SO, and acifluorfen (compared to sensitive lines) and they also possessed higher levels o f (Cu/Zn) superoxide dismutase and glutathione reductase. The tolerance to oxidant stresses in C onyza and maize increases with plant age, suggesting that the shift to resistance is a constitutive, earlier expression o f the genes normally expressed later in development.
	Reference	Z. Naturforsch. 45c, 463 (1990); received November 9 1989
	Published	1990
	Keywords	Paraquat, Acifluorfen, Atrazine, Photoinhibition, Superoxide Dismutase
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	TEI-XML for	default:Reihe_C/45/ZNC-1990-45c-0463.pdf
	Identifier	ZNC-1990-45c-0463
	Volume	45

3	Author	Susana Shochata, Noam Adira, Alma Gala, Yorinao Inoueb, Laurence Metsc, Itzhak Ohada	Requires cookie*
	Title	Photoinactivation of Photosystem II and Degradation of the D 1 Protein are Reduced in a Cytochrome b j f -Less Mutant of Chlamydomonas reinhardtii
	Abstract	The effect of unoccupancy of the Q B site by plastoquinone on the photoinactivation of reac tion center II in a Cyt b jf-less mutant of Chlamydomonas reinhardtii, Bb, was investigated. In these cells the oxidation of plastoquinol generated by electron flow via RC II to plastoquinone and thus the turnover of PQH2/PQ via the Q B site are drastically reduced. Reaction center II of the mutant cells was resistant to photoinactivation relative to the control cells as demonstrated by measurements of light-induced destabilization of S2-QB charge recombination, rise in in trinsic fluorescence and loss of variable fluorescence. These parameters relate to functions in volving the reaction center II D 1 protein. The light-induced degradation of D 1 in the mutant cells was also considerably reduced, with a ;l/ 2 value of 7 h as compared, under similar condi tions, to about 1.5 h for the control cells. These results indicate that the photoinactivation of RC II and turnover of the D 1 protein are related and require occupancy of the Q B site by PQ and its light-driven reduction.
	Reference	Z. Naturforsch. 45c, 395—401 (1990); received November 24 1989
	Published	1990
	Keywords	Cytochrome b j f, Chlamydomonas, D 1 Turnover, Q B, Thermoluminescence, Photoinhibition
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	TEI-XML for	default:Reihe_C/45/ZNC-1990-45c-0395.pdf
	Identifier	ZNC-1990-45c-0395
	Volume	45