Electron paramagnetic resonance (EPR) and electron-nuclear double resonance studies of the photosystem (PS) I quinone acceptor, A1, in phylloquinone biosynthetic pathway mutants are described. Room temperature continuous wave EPR measurements at X-band of whole cells of menA and menB interruption mutants show a transient reduction and oxidation of an organic radical with a g-value and anisotropy characteristic of a quinone. In PS I complexes, the continuous wave EPR spectrum of the photoaccumulated Q- radical, measured at Q-band, and the electron spin-polarized transient EPR spectra of the radical pair P700+ Q-, measured at X-, Q-, and W-bands, show three prominent features: (i) Q- has a larger g-anisotropy than native phylloquinone, (ii) Q- does not display the prominent methyl hyperfine couplings attributed to the 2-methyl group of phylloquinone, and (iii) the orientation of Q- in the A1 site as derived from the spin polarization is that of native phylloquinone in the wild type. Electron spin echo modulation experiments on P700+ Q- show that the dipolar coupling in the radical pair is the same as in native PS I, i.e. the distance between P700+ and Q-(25.3 ± 0.3 Å) is the same as between P700+ and A1 - in the wild type. Pulsed electron-nuclear double resonance studies show two sets of resolved spectral features with nearly axially symmetric hyperfine couplings. They are tentatively assigned to the two methyl groups of the recruited plastoquinone- 9, and their difference indicates a strong inequivalence among the two groups when in the A1 site. These results show that Q (i) functions in accepting an electron from A0 - and in passing the electron forward to the iron-sulfur clusters, (ii) occupies the A1 site with an orientation similar to that of phylloquinone in the wild type, and (iii) has spectroscopic properties consistent with its identity as plastoquinone-9.
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