Recruitment of a foreign quinone 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 A, site of p hotosystem I with a number of experimental indicators identifying Q as plas toquinone-9, A global multiexponential analysis of time-resolved optical sp ectra in the blue region shows the following three kinetic components: 1) a 3-ms lifetime in the absence of methyl viologen that represents charge rec ombination between P700(+) and an FeS- cluster; 2) a 750-mu s lifetime that represents electron donation from an FeS- cluster to methyl viologen; and 3) an similar to 15-mu s lifetime that represents an electrochromic shift o f a carotenoid pigment. Room temperature direct detection transient EPR stu dies of forward electron trans fer show a spectrum of P700(+) Q(-) during t he lifetime of the spin polarization and give no evidence of a significant population of P700(+) FeS- for t less than or equal to 2-3 mu s. The UV dif ference spectrum measured 5 mu s after a flash shows a maximum at 315 nm, a crossover at 280 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 radica l, Kinetic measurements that monitor Q at 315 nm show a major phase of forw ard electron transfer to the FeS clusters with a lifetime of similar to 15 mu s, which matches the electrochromic shift at 485 nm of the carotenoid, a s well as an minor phase with a lifetime of similar to 250 mu s. Electromet ric measurements show similar biphasic kinetics, The slower kinetic phase c an be detected using time-resolved EPR spectroscopy and has a spectrum char acteristic of a semiquinone anion radical. We estimate the redox potential of plastoquinone-9 in the A(1) site to be more oxidizing than phylloquinone so that electron transfer from Q(-) to F-X is thermodynamically unfavorabl e in the menA and menB mutants.