ИСТИНА

The role of the accessory chlorophyll molecules (Chl2A and Chl2B or A-1A and A-1B) in Photosystem I (PS I) is not clearly understood. In this work we present the results which clarify the significance of the Chl2A and Chl2B chlorophylls in electron transfer. In this study we used two pairs of mutants in the cyanobacterium Synechocystis sp. PCC 6803 with mutations of the asparagine residues N591PsaB and N604PsaA associated with Chl2A/2B in the A- and B-branches of PS I, respectively. We investigated the consequences of exchanging the asparagine residue with leucine (ANL/BNL) and histidine (ANH/BNH) residues using steady-state and pump-probe femtosecond differential spectroscopy. Femtosecond spectroscopy allowed to us resolve the spectra of the electron transfer intermediates P700+A0- and P700+A1-. The P700+A0- spectra were similar in the mutants and in the wild type (WT), which indicated that primary charge separation was not affected. In the spectra of P700+A1-, as well as in the steady-state differential (oxidized minus reduced) absorption spectra of the A-branch mutants, we observed a significant decrease of the amplitude of the bleaching at 700 nm in comparison with the WT, whereas the difference between the WT and B-branch mutants was much less pronounced. These results suggest that accessory chlorophylls Chl2A and Chl2B are involved in electron transfer as a part of the primary electron acceptor A0, which represents a dimer of chlorophylls Chl2A/Chl3A and Chl2B/Chl3B. Our data also confirm the preferential electron transfer from A0 to A1 via A-branch of the cofactors at room temperature.