chlorophyll-a and ioxynil

The effects of short-term treatment with phenylurea (DCMU, isoproturon) and phenol-type (ioxynil) herbicides on the green alga Chlorella kessleri and the cyanobacterium Synechocystis salina with different organizations of photosystem II (PSII) were investigated using pulse amplitude modulated (PAM) chlorophyll fluorescence and photosynthetic oxygen evolution measured by polarographic oxygen electrodes (Clark-type and Joliot-type). The photosynthetic oxygen evolution showed stronger inhibition than the PSII photochemistry. The effects of the studied herbicides on both algal and cyanobacterial cells decreased in the following order: DCMU>isoproturon>ioxynil. Furthermore, we observed that the number of blocked PSII centers increased significantly after DCMU treatment (204-250 times) and slightly after ioxynil treatment (19-35 times) in comparison with the control cells. This study suggests that the herbicides affect not only the acceptor side but also the donor side of PSII by modifications of the Mn cluster of the oxygen-evolving complex. We propose that one of the reasons for the different PSII inhibitions caused by herbicides is their influence, in different extents, on the kinetic parameters of the oxygen-evolving reactions (the initial S0-S1 state distribution, the number of blocked centers SB, the turnover time of Si states, misses and double hits). The relationship between the herbicide-induced inhibition and the changes in the kinetic parameters is discussed.

Topics: Chlorella; Chlorophyll; Diuron; Dose-Response Relationship, Drug; Electron Transport; Herbicides; Iodobenzenes; Nitriles; Oxygen; Phenylurea Compounds; Photosynthesis; Photosystem II Protein Complex; Synechocystis

A new Synechocystis 6714 mutant, IoxIIA, resistant to the phenol-type herbicide ioxynil was isolated and characterized. The mutation found in the psbA gene (encoding the D 1 photosystem II protein) is at the same codon 266 as for the first ioxynil-resistant mutant IoxIA previously selected [G. Ajlani, I. Meyer, C. Vernotte, and C. Astier, FEBS Lett. 246, 207-210 (1989)]. In IoxIIA, the change of Asn 266 to Asp gives a 3 x resistance, whereas in IoxIA, the change of the same amino acid to Thr gives a 10 x resistance. The effect of these different amino acid substitutions on the ioxynil resistance phenotype has allowed us to construct molecular models and calculate the hydrogen-bonding energies between the hydroxyl group of ioxynil and the respective amino acids at position 266.

Topics: Amino Acid Sequence; Chlorophyll; Cloning, Molecular; Cyanobacteria; Drug Resistance; Genes, Plant; Herbicides; Iodobenzenes; Light-Harvesting Protein Complexes; Models, Molecular; Molecular Sequence Data; Mutation; Nitriles; Photosynthetic Reaction Center Complex Proteins; Photosystem II Protein Complex; Plant Proteins; Protein Conformation

A Synechocytis 6714 mutant resistant to the phenol-type herbicide ioxynil was isolated and characterized. Sensitivity to DCMU and atrazine was tf measured in whole cells and isolated thylakoids. The mutant presents the same sensitivity to atrazine as the wild type and a slightly increased sensitivity to DCMU. A point mutation has been found at codon 266 in the psbAI coding locus (AAC to ACC) resulting in an amino acid change from asparagine to threonine in the D1 protein.

Topics: Amino Acid Sequence; Base Sequence; Chlorophyll; Cloning, Molecular; Codon; Cyanobacteria; Drug Resistance, Microbial; Genes; Herbicides; Iodobenzenes; Light-Harvesting Protein Complexes; Molecular Sequence Data; Mutation; Nitriles; Photosynthetic Reaction Center Complex Proteins; Plant Proteins