Recruitment of a foreign qiunone into the a(1) site of photosystem I: Altered kinetics of electron transfer in phylloquinone biosynthetic pathway mutants studied by time-resolved optical, EPR, and electrometric techniques

Interruption of the menA or menB gene in Synechocystis sp. PCC 6803 results in the incorporation of a foreign quinone, termed Q, into the A₁ site of photosystem I with a number of experimental indicators identifying Q as plastoquinone-9. A global multiexponential analysis of time-resolved optical spectra in the blue region shows the following three kinetic components: 1) a 3-ms lifetime in the absence of methyl viologen that represents charge recombination between P700⁺ and an FeS^- cluster; 2) a 750-μs lifetime that represents electron donation from an FeS^- cluster to methyl viologen; and 3) an ~15-μs lifetime that represents an electrochromic shift of a carotenoid pigment. Room temperature direct detection transient EPR studies of forward electron transfer show a spectrum of P700⁺ Q^- during the lifetime of the spin polarization and give no evidence of a significant population of P700⁺ FeS^- for t ≤ 2-3 μs. The UV difference spectrum measured 5 μs after a flash shows a maximum at 315 nm, a crossover at 286 nm, and a minimum at 255 nm as well as a shoulder at 290-295 nm, all of which are characteristic of the plastoquinone-9 anion radical. Kinetic measurements that monitor Q at 315 nm show a major phase of forward electron transfer to the FeS clusters with a lifetime of ~15 μs, which matches the electrochromic shift at 485 nm of the carotenoid, as well as an minor phase with a lifetime of ~250 μs. Electrometric measurements show similar biphasic kinetics. The slower kinetic please can be detected using time-resolved EPR spectroscopy and has a spectrum characteristic of a semiquinone anion radical. We estimate the redox potential of plastoquinone-9 in the A₁ site to be more oxidizing than phylloquinone so that electron transfer from Q^- to F(x) is thermodynamically unfavorable in the menA and menB mutants.