Biochemical and biophysical characterization of photosystem I from phytoene desaturase and ζ-carotene desaturase deletion mutants of Synechocystis Sp. PCC 6803: Evidence for PsaA- and PsaB-side electron transport in cyanobacteria

In photosystem I, oxidation of reduced acceptor A₁^- through iron-sulfur cluster F_X is biphasic with half-times of ∼5-30 ns ("fast" phase) and ∼150-300 ns ("slow" phase). Whether these biphasic kinetics reflect unidirectional electron transfer, involving only the PsaA-side phylloquinone or bi-directional electron transfer, involving both the PsaA- and PsaB-side phylloquinones, has been the source of some controversy. Brettel (Brettel, K. (1988) FEBS Lett. 239, 93-98) and Joliot and Joliot (Joliot, P., and Joliot, A. (1999) Biochemistry 38, 11130-11136) have attributed to nearby carotenoids electrochromic band shifts, accompanying A₁ reduction, centered at ∼450 and 500-510 nm. As a test of these assignments, we separately deleted in Synechocystis sp. PCC 6803 the genes that encode phytoene desaturase (encoded by crtP (pds)) and ζ-carotene desaturase (encoded by crtQ (zds)). The pds^- and zds^- strains synthesize phytoene and ζ-carotene, respectively, both of which absorb to shorter wavelength than β-carotene. Compared with wild type, the mutant A₁^-(FeS) - A₁(FeS) ^- difference spectra, measured in cells and photosystem I complexes, retain the electrochromic band shift centered at 450 nm but show a complete loss of the electrochromic band shifts centered at 500-510 nm. Thus, the latter clearly arise from β-carotene. In the wild type, the electrochromic band shift of the slow phase (centered at 500 nm) is shifted by 6 nm to the blue compared with the fast phase (centered at 506 nm). Thus, the carotenoid pigments acting as electrochromic markers during the fast and slow phases of A ₁^- oxidation are different, indicating the involvement of both the PsaA- and the PsaB-side phylloquinones in photosystem I electron transport.