PARENT SESSION
Posters P4Ab Type II reaction centres: Acceptor side. Abstracts (272-288)

Influences of PSII herbicides on the T4 mutant of Blastochloris viridis. Christian Fufezan^{*,1, 2}, Friedel Drepper¹, Hanno Juhnke³, Roy Lancaster³, Alfred Rutherford², Anja Krieger-Liszkay¹, ¹ Institut für Biologie II, Biochemie der Pflanzen, Freiburg, Germany² Section de Bioénergétique, Gif-sur-Yvette, France³ Max Planck Institute of Biophysics, Frankfurt am Main, Germany

ABSTRACT-

In PSII urea (e.g. DCMU) and phenolic (e.g. bromoxynil) herbicides have opposite effects on the redox potential (E_m) of Q_A and this has been related to their mode of action [1]. These herbicides do not effect purple bacterial reaction centres, however the terbutryn-resistant T4 mutant (YL222F) is the exception to this rule [2]. We have studied the effect of these herbicides in the T4 mutant using redox potentiometry and time-resolved absorption spectroscopy from 300 to 80K. At room temperature (293K) the recombination kinetics in the presence of bromoxynil was faster than in the presence of DCMU. Two phases (fast and slow) of P⁺Q_A^- recombination were attributed to two different populations of reaction centres in different states, in accordance with the literature [3,4]. The herbicides did induce small differences in the activation barriers however these did not explain the herbicide-induced differences in the kinetics at 293K. Instead these were attributed to a change in the relative amplitude of the phases, with the fast:slow ratio being ∼9:1 with bromoxynil and ∼5:5 with DCMU. Redox titrations +/- herbicide were done by monitoring the reduced Q_A via the presence of the ∼3ns P⁺Pheo^- recombination kinetic at 1300 nm [5]. The value obtained in the absence of herbicides (-53±10 mV at pH 6.3, -110±15 mV at pH 7.7) were similar to those reported for the wild-type [6]. At pH 6.3 and in the presence of bromoxynil the E_m was shifted by -50±20 mV, while in the presence of DCMU the E_m was shifted by +50±20 mV. As the titrations were done over a time-range that is assumed to be much slower than the protonation equilibrium responsible for the two populations, the potential measured is considered to be a weighted average of two potentials for Q_A. The influence of the herbicides can be considered to be on the pK that controls the interconversion of the two reaction centre forms. The effects seen in PSII may reflect similar phenomena.

[1] Rutherford, A.W. and Krieger-Liszkay, A. (2001), TiBS 26, 648-653.

[2] Sinning, I. et al. (1989), Biochemistry 28, 5544-5553.

[3] Shopes, R.J. and Wraight, C.A. (1987), Biochim Biophys. Acta 1057, 109-114.

[4] Sebban, P. and Wraight, C.A. (1989), Biochim Biophys. Acta 974, 54-65.

[5] Gibasiewicz, K. et al. (1999), Chem. Phys. Lett. 315, 95-102.

[6] Prince et al. (1976), Biochim Biophys. Acta 440, 622-636.

KEY WORDS: t4, protein substates, herbicides, viridis