Steady-state kinetic analyses of photosystem II activity catalyzed by lipophilic electron acceptors

Interaction of 2-n-heptyl-4-hydroxyquinoline-N-oxide with photosystem II in chloroplasts and subchloroplast particles

The effects of 2-n-heptyl-4-hydroxyquinoline-N-oxide on electron transport in thylakoids and oxygen-evolving photosystem II particles has been examined. Kinetic fluorescence studies reveal that the site of inhibition for alkyl derivatives of hydroxyquinoline-N-oxide (I50 approximately equal to 2 microM) is located between Q and plastoquinone. Studies with thylakoids isolated from atrazine-resistant pigweed plants indicate that the modification in the Q/B membrane complex that confers increased resistance to inhibition by atrazine also results in decreased sensitivity to inhibition by 2-n-heptyl-4-hydroxyquinoline-N-oxide (resistant/ sensitive ratio = 11). From the results of tetramethylphenylenediamine by-pass experiments, determinations of inhibitor sensitivity in trypsin-treated thylakoids and competitive displacement experiments made with [14C]metribuzin in thylakoids and photosystem II particles, it is suggested that 2-n-heptyl-4-hydroxyquinoline-N-oxide binds in a region of the Q/B complex that is distinct from the 3-(3,4-dichloro)-1,1-dimethyl urea and atrazine binding sites.

Electron acceptance at photosystem II in an oxygen-evolving photosystem II preparation: clear discrimination of two sites of electron acceptance for quinones and quinonediimines associated with a photosystem II preparation

Oxygen-producing electron transport reactions of a photosystem II preparation are described. The preparation has six major peptides with apparent molecular weights of 36,000, 31,000, 28,000, 27,000, 25,000, and 21,000. Sucrose density gradient centrifugation indicates that the preparation is more homogeneous and more dense than control thylakoid membranes. The preparation photoreduces a number of known photosystem II oxidants including the Class I acceptor, ferricyanide; the Class II acceptor, 2,6-dichloroindophenol; and the Class III acceptor, 2,6-dichlorobenzoquinone. However, quinonediimines such as phenylenediimine are not reduced, suggesting that these substances are reduced at sites in the thylakoid membrane which are not found in the photosystem II preparation. All the oxygen-producing reactions are sensitive to inhibition by 3-(3,4-dichlorophenyl)-1,1-dimethylurea.

Metronidazole and the isolation of temperature-sensitive photosynthetic mutants in cyanobacteria

A procedure has been developed for use of metronidazole (2-methyl-5-nitroimidazole-1-ethanol) as an enrichment agent during the isolation of temperature-sensitive, photosynthetic mutants in the cyanobacterium Synechococcus cedrorum. The protocol includes incubation with this drug following mutagenesis with N-methyl-N'-nitro-N-nitrosoguanidine. Incubation of photosynthetically active S. cedrorum cells with 1 mM metronidazole causes a light-dependent reduction of cell viability. Maximum reduction in cell viability occurred following 6 h of incubation. Cessation of electron transport reduced the impact of the drug by five orders of magnitude. Yet during the time of incubation, metronidazole did not influence the electron transport capacities of the S. cedrorum cells, suggesting that the thylakoid membrane was not the target of the toxic effects of this drug. In addition, this drug was found to be an effective electron acceptor to photosystem I although high concentrations were required to observe maximum rates of electron transfer. Metronidazole interacted in a noncompetitive manner with methyl viologen, which suggested that those two acceptors to photosystem I have unique reduction sites on the S. cedrorum thylakoid membrane. The temperature-sensitive strains that were isolated using the procedure presented here were assessed for photosynthetic electron transport and chlorophyll fluorescence (induction kinetics and low-temperature emission spectra) characteristics. Approximately one-half of the temperature-sensitive mutants isolated possessed abnormal photosynthetic properties when shifted to the restrictive temperature (40 degrees C). A total of 31 throughout the photosynthetic electron-transport chain.