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.
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Creuzet et al. (1990) studied this question.
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