Evidence from time resolved studies of the P700  /A 3 1 radical pair for photosynthetic electron transfer on both the PsaA and PsaB branches of the photosystem I reaction centre I.P. Muhiuddin a , P. Heathcote b , S. Carter a , S. Purton a , S.E.J. Rigby b , M.C.W. Evans a ; * a Department of Biology, University College London, Gower St., London WC1E 6BT, UK b School of Biological Sciences, Queen Mary and West¢eld College, Mile End Road, London E1 4NS, UK Received 4 July 2001; accepted 9 July 2001 First published online 24 July 2001 Edited by Richard Cogdell Abstract Kinetic analysis using pulsed electron paramagnetic resonance (EPR) of photosynthetic electron transfer in the photosystem I reaction centres of Synechocystis 6803, in wild- type Chlamydomonas reinhardtii, and in site directed mutants of the phylloquinone binding sites in C. reinhardtii, indicates that electron transfer from the reaction centre primary electron donor, P700, to the iron^sulphur centres, Fe^S X=A=B , can occur through either the PsaA or PsaB side phylloquinone. At low temperature reaction centres are frozen in states which allow electron transfer on one side of the reaction centre only. A fraction always donates electrons to the PsaA side quinone, the remainder to the PsaB side. ß 2001 Federation of European Biochemical Societies. Published by Elsevier Science B.V. All rights reserved. Key words: Electron transfer ; Photosystem I reaction centre ; Synechocystos 6803; Chlamydomonas reinhardtii 1. Introduction The primary events of photosynthetic energy conversion occur in membrane bound protein complexes termed reaction centres. Although the basic mechanism of energy conversion is similar in all reaction centres, two types of reaction centre have been identi¢ed which di¡er in the redox potential range and mechanism of electron transfer from the reaction centre. Photosystem I of oxygenic organisms and the reaction centres of green sulphur bacteria and Heliobacteria (Type I) reduce a low potential ferredoxin, at 3400 mV. Photosystem II of oxy- genic organisms and purple bacterial reaction centres (Type II) reduce a quinone with a redox potential of around 0 mV which transfers electrons to the cytochrome complex of the electron transfer chain. High resolution structures of examples of both types of reaction centre are now available [1,2]. In both types the core of the reaction centre is a dimeric struc- ture supporting the electron donor chlorophyll dimer, four other redox active chlorins and two quinones. The path of electron transfer in purple bacterial reaction centres is well established. Electron transfer is unidirectional using only one side of the reaction centre. The second side of the reaction centre also has a potential electron transfer pathway with the intermediary bacteriochlorophyll and bacteriopheophytin, but is not functional [3]. The mechanism which directs the elec- tron along the active path is not known. Photosystem II func- tions in the same way. Type I reaction centres are analogous to Type II centres in the reaction centre chlorin quinone part of the structure, they di¡er in having three iron^sulphur centres (Fe^S A=B=X ) as the terminal acceptors. Fe^S X is suspended between the two ma- jor polypeptides (PsaA and PsaB) while Fe^S A=B which func- tion as the terminal membrane bound electron acceptors are associated with a small peripheral protein (PsaC). Electrons are transferred from the quinone to Fe^S X and then Fe^S A=B . Although the electron transfer components of the photosys- tem I reaction centre are well characterised, it has been di¤- cult to unequivocally de¢ne the route of electron transfer and the kinetic properties of the reaction centre. Experiments us- ing either optical or electron paramagnetic resonance (EPR) measurements of A 3 1 together with biochemical or genetic modi¢cation of the PSI iron^sulphur centres established an electron transfer path from A 1 to Fe^S X with an electron transfer rate at room temperature of t 1=e W200 ns [4^6]. We have recently used site directed mutagenesis of the conserved tryptophan residue PsaAW693 to show that this rate is asso- ciated with the phylloquinone bound to PsaA [7]. It has gen- erally been thought that by analogy with the Type II reaction centres electron transfer would be unidirectional in photosys- tem I, and that this route of electron transfer via PsaA would therefore be the only path of electron transfer. However it has been reported that a faster rate of electron transfer from A 1 to Fe^S X of W20 ns could also be detected [8]. It was also found that mutations of the phylloquinone binding sites on either PsaB (PsaBW673F) or PsaA (PsaAW693H/L) did not prevent photoautotrophic growth, although the PsaA mutations did make the cultures oxygen sensitive [7,10]. Joliot and Joliot [9] have recently developed optical techniques which allow them to measure the rate of phylloquinone oxidation in whole cells of the green alga Chlorella pyrenoidosa. They observe two rates of oxidation, t 1=e = 13 and 140 ns at room temperature. The two phases are of approximately equal intensity. They suggest that electron transfer is in fact bidirectional, initial electron transfer is randomly directed to either side of the reaction centre with the overall rate limited on each side by 0014-5793 / 01 / $20.00 ß 2001 Federation of European Biochemical Societies. Published by Elsevier Science B.V. All rights reserved. PII:S0014-5793(01)02696-5 *Corresponding author. Fax: (44)-20-7679 7312. E-mail address: mike.evans@ucl.ac.uk (M.C.W. Evans). Abbreviations : EPR, electron paramagnetic resonance; ESE, electron spin echo; ESP, electron spin polarisation; P700, reaction centre chlorophyll of photosystem I; A 1 , phylloquinone electron carrier in photosystem I; Fe^S A=B=X , iron^sulphur centre electron acceptors in the photosystem I reaction centre FEBS 25105 FEBS Letters 503 (2001) 56^60