Biochemical and biophysical characterization of photosystem I from phytoene desaturase and ξ-carotene desaturase deletion mutants of Synechocystis sp PCC 6803

In photosystem I, oxidation of reduced acceptor A(1) - through iron-sulfur cluster F-X is biphasic with half-times of similar to 5-30 ns ("fast" phase) and similar to 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 A(1) reduction, centered at similar to 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 zeta-carotene desaturase ( encoded by crtQ (zds)). The pds(-) and zds(-) strains synthesize phytoene and zeta-carotene, respectively, both of which absorb to shorter wavelength than zeta-carotene. Compared with wild type, the mutant A(1)(-)(FeS) similar to A(1)(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 beta-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.