Delection of the PsaF Polypeptide Modifies the Environment of the Redox-Active Phylloquinone (A_1). Evidence for Unidirectionality of Electron Transfer in Photosystem I

摘要

The issue of whether one or both branches of electron-transfer cofactors is active n Photosystem I (PSI) was studied using a strategy employing interposon mutagenesis and electron paramagnetic resonance (EPR) spectroscopy. PS I complexes were isolated using n-dodecyl-β-D-multoside (DM) or Triton X-100 (TX-100) from wild-type and mutant strains of Synechococcus sp. PCC 7002 lacking specific PS I polypeptides. The principal values of the g-tensor of A_1 were determined by Q-band (34.0 GHz) EPR spectroscopy of perdeuterated, wild-type PSI complexes as g_(xx) = 2.0062, g_(yy) = 2.0050, and g_(zz) = 2.0021, and a stoichiometry of ≤1.0 A_1~- per P700~+ was measured by spin quantitation of photoaccumulated, wild-type PS I complexes at illumination temperatures between 195 and 220 K. The characteristic anisotropic EPR spectrum of A_1~- was photoaccumulated in all mutant PS I complexes isolated with DM and most mutant PS I complexes isolated with TX-100; however, A_0~- was photoaccumulated in PS I complexes isolated with TX-100 from the psaF and psaE psaF mutants. PS I complexes isolated with TX-100 from the wild type and the psaE psaF mutant retained 2.5 phylloquinone (PhQ) and 2.6 PhQ per 100 Chl, respectively, indicating that the failure to observe A_1~- in the psaE psaF mutant is not due to the loss PhQ. Steady-state rates of light-induced flavodoxin reduction for PS I complexes isolated with DM and TX-100 from the psaF and psaE psaF mutants were nearly identical and differed by less than a factor of 2 from that for the wild-type. The inability to photoaccumulate a second A_1~- in the wild-type and the psaE psaF mutant indicates that only one of the two resident PhQs in PS I is redox active. The presence of A_0~- in the psaE and psaE psaF mutants is explained by the protonation and secondary reduction of the semiquinone anion radical of PhQ which is made solvent-accessible by removal of the PsaF polypeptide and by exposure to TX-100. Since only the loss of the C_3-distal PsaF or PsaE and PsaF polypeptides affects the spectroscopic properties of A_1, the binding site of the redox-active PhQ is associated with the nonprimed α-helices assigned to PsaA/PsaB the electron density map.