ИСТИНА

During the past decade the electron transfer processes in photosystem I (PS I) from Synechocystis sp. PCC 6803 were mainly investigated under single-flash excitation. The big progress has been achieved in this field, and several kinetic models, describing these reactions were suggested [1, 2]. However the mechanisms of the electron transfer in PS I under continuous illumination are still unknown. Only few models were presented to date [3]. Here we present the results of the investigation of the dependence of the balance of the P700 oxidation and reduction reactions under steady-state conditions. The P700 redox-state changes were observed under continuous white-light illumination using X-band transient EPR. The FA/FB-depleted (FX-core) complexes of PS I were used in order to decrease the rate of the P700+ oxidation. 2,6-dichlorophenolindophenol (DCPIP) at different concentrations in the presence of the excess of sodium ascorbate was used as the mediator of cyclic electron transfer [4]. In the presence of the low concentration of DCPIP the P700 oxidation is already very fast, it takes several seconds at the lowest DCPIP concentration, while the dark P700+ reduction initially occur in the minute time range. Upon the increase in the mediator concentration the rate of the P700+ reduction accelerates much faster, than the rate of the P700 oxidation. This balance of the oxidation and reduction of P700 could be estimated as the amplitude of the steady-state P700+ signal, which significantly drops upon the increase of the DCPIP concentration. The data clearly shows the difference in the mechanisms of the electron transfer in PS I complexes under single-flash excitation and under continuous illumination.