PARENT SESSION
Posters P2A Type I reaction centres. Abstracts (181-218)

Directionality of electron transport through Photosystem I of Chlamydomonas reinhardtii probed by transient electron paramagnetic resonance. Michael McConnell^*,1, V.M. Ramesh¹, Ingelög Wyndhamn², Art van der Est², Andrew Webber¹, ¹ School of Life Sciences and Center for the Study of Early Events in Photosynthesis, Tempe, Arizona, USA² Department of Chemistry, St. Catharines, Ontario, Canada

ABSTRACT- Methionines MetA684 and MetB664 in Chlamydomonas reinhardtii act as ligands to the central Mg of the chlorophylls eC-A3 and eC-B3 in each respective branch of the two possible electron transfer chains in PSI. One or both of these chlorophylls are assumed to be A₀ the first spectroscopically detectable electron acceptor. Changing the ligating residue is expected to affect the ability for the chlorophyll to participate in electron transport following exciton trapping and recently, an ultrafast absorption difference study suggested nearly equal activity of the two branches and gave evidence that the histidine mutations blocked electron transfer at A₀, preventing reduction of the A₁ acceptor in the branch containing the mutation^a. Another study in which the out-of-phase EPR echo signal was measured also suggested that the histidine mutants blocked electron transfer^b. In contrast, a recent EPR study^c of leucine mutants of the corresponding methionines in cyanobacteria, indicated a strong asymmetry in favour of electron transfer in the PsaA branch and only partial blockage of electron transfer to A₁ in the PsaA mutant. In this study we have employed the same techniques used for the cyanobacteria studies to study the histidine A₀ mutants in Chlamydomonas. At low temperature our data suggest that despite the His mutation in the A-branch cyclic electron transfer from P700^* to A₁ still occurs in a fraction of the reaction centres. This is demonstrated by the presence a weak P700⁺A₁^- radical pair signal in the MHA684 mutant. In the MHB664 mutant the P700⁺A₁^- signal is maintained at similar levels to the wild type. Interestingly, the low temperature CW EPR spectra indicate an accumulation of reduced acceptor, possibly A₀^-, occurs in both mutants regardless of their relative electron transport capabilities. These data will be discussed in relationship to our current understanding of a model for the directionality of electron transport in PSI on a species-specific basis. a - Ramesh et al., 2003. Biochemistry, 43: 1369-75. b - Fairclough et al., 2003. BBA, 1606: 43-55. c - Cohen et al., 2003. Biochemistry, 43: 4741-54.

KEY WORDS: electron branch migration, photosystem I, EPR, Chlamydomonas