PARENT SESSION
Posters P2A Type I reaction centres. Abstracts (181-218)

Methionine to asparagine mutations of the A_o chlorophyll ligand in the PsaA and PsaB subunits of Photosystem I in Synechocystis sp. PCC 6803. Rachel Cohen^*,1, Wu Xu², Parag Chitnis², John Golbeck¹, ¹ 1 Department of biochemistry and Molecular Biology, The Pennsylvania State University, University Park, PA, 16802, USA² Department of Biochemistry, Molecular Biology, and Biophysics, Iowa State University, Ames, IA, 50010, USA

ABSTRACT- The PsaA and PsaB proteins that form the heterodimeric core of Photosystem I (PS I) are related by a pseudo-C₂ symmetry axis. The electron transfer cofactors, beginning with P₇₀₀, and continuing with the primary and secondary electron acceptors, A₀ and A₁, are located on redundant branches that converge at the F_X iron-sulfur cluster. This bifurcating chain begs the question of whether the electron utilizes one branch or both branches. There is a general consensus that the PsaA branch is active, but the question is the extent to which the PsaB branch is active. To address this issue, we constructed mutants in which the axial methionine ligand on A₀ was replaced with asparagine on the PsaA (amn) and PsaB (bmn) branches. The flash-induced absorbance change of P₇₀₀ in PS I complexes from the bmn mutant was identical to that of the wild-type, but the flash-induced absorbance change in the amn mutant was only 40% that of the wild-type. Thus, the quantum yield of electron transfer is affected only on the PsaA side and not on the PsaB side. Interestingly, the charge recombination kinetics of P₇₀₀⁺ were accelerated from ca. 150 ms in the wild-type to ca. 2 ms in the bmn mutants. EPR spectroscopy indicates that the amount of F_A and F_B reduced by dithionite is the same in the wild-type, amn and bmn mutants. Yet, when the samples are frozen under illumination at 15 K, the amount of F_A and F_B reduced is significantly less in the bmn mutant. While the amn mutant shows expected results that correlate with PsaA branch activity in cyanobacterial PS I, the bmn mutant hints to a possible problem in electron transfer between F_X and F_A/F_B. These results are discussed in terms of PsaC binding to the PS I core. (Funded by NSF to J.H.G.).

KEY WORDS: site directed mutants, photosystem I, electron transfer, iron sulfur clusters