The response of electron transport mediated by active NADPH dehydrogenase complexes to heat stress in the cyanobacterium Synechocystis 6803.

The electron-transport machinery in photosynthetic membranes is known to be very sensitive to heat. In this study, the rate of electron transport (ETR) driven by photosystem I (PSI) and photosystem II (PSII) during heat stress in the wild-type Synechocystis sp. strain PCC 6803 (WT) and its ndh gene inactivation mutants DeltandhB (M55) and DeltandhD1/ndhD2 (D1/D2) was simultaneously assessed by using the novel Dual-PAM-100 measuring system. The rate of electron transport driven by the photosystems (ETR(PSs)) in the WT, M55, and D1/D2 cells incubated at 30 degrees C and at 55 degrees C for 10 min was compared. Incubation at 55 degrees C for 10 min significantly inhibited PSII-driven ETR (ETR(PSII)) in the WT, M55 and D1/D2 cells, and the extent of inhibition in both the M55 and D1/D2 cells was greater than that in the WT cells. Further, PSI-driven ETR (ETR(PSI)) was stimulated in both the WT and D1/D2 cells, and this rate was increased to a greater extent in the D1/D2 than in the WT cells. However, ETR(PSI) was considerably inhibited in the M55 cells. Analysis of the effect of heat stress on ETR(PSs) with regard to the alterations in the 2 active NDH-1 complexes in the WT, M55, and D1/D2 cells indicated that the active NDH-1 supercomplex and mediumcomplex are essential for alleviating the heat-induced inhibition of ETR(PSII) and for accelerating the heat-induced stimulation of ETR(PSI), respectively. Further, it is believed that these effects are most likely brought about by the electron transport mediated by each of these 2 active NDH-1 complexes.