Molecular Mechanism of Adaptation to photo-Oxidative Stresses in Cyanobacteria.

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1. Ascorbate peroxidase (APX) was found in six species of cyanobacteria among ten speceis tested. Upon the addition of H_2 ^<18>O_2 to the cells of APXcontaining cyanobacteria, ^<16>O_2 derived from water and ^<18>O_2 derived from H_2 ^<18>0_2 were evolved in the light. However, the evolution of ^<16>O_2 was not observed in APX-lacking cyanobacteria. Furthermore, the quenching of chlorophyll fluorescence induced by hydrogen peroxide was observed only in APX-containing cyanobacteria. Thus, cyanobacteria can be devided into two groups, those that has and those that lacks APX.2. Synechococcus PCC 7002 showed low oxidation level of P700 under far-red light(FR)at 6 W/m^2, which induced the nearly complete oxidation of P700 of spinach leaves, and a stronger FR was required to observe the oxidation of P700. DCMU did not affect the level of P700^+, but DBMIB induced the oxidation of P700 under the far-red light, indicating that the donation site to the intersystem chain by cyclic and respiratory flows is the plastoquinone. The cyclic and respiratory flows as observed by P700 redox change were inhibited by HgCl^2, but not by antimycin A and rotenone, showing the operation of a different cyclic electron flow in Synechococcus from that in plant chloroplasts.3. Redox behaviors of P700 in the wild type and an NAD(P)H dehydrogenase(NDH)deficient mutant of Synecho6ystis PCC 6803 upon illumination of FR indicate that reduction of plastoqunone pool in the thylakoid membrane by electrons from respiratory substrates results in the low oxidation level of P700 under FR. P700 redox kinetics induced by flash light and FR showed that NDH is the main input to the plastoquinone pool for cyclic flow as well as for respiration.