Loss of the cytochrome b6f subunit PetN destabilizes the complex and severely impairs state transitions in Anabaena variabilis

The cytochrome b6f complex (Cyt b6f) plays pivotal roles in both linear and cyclic electron transport of oxygenic photosynthesis in plants and cyanobacteria. The four large subunits of Cyt b6f are responsible for organizing the electron transfer chain within Cyt b6f and have their counterparts in the cytochrome bc1 complex in other bacteria. The four small subunits of Cyt b6f are unique to oxygenic photosynthesis, and their functions remain to be elucidated. Here, we report that Cyt b6f was destabilized by the loss of PetN, one of the small subunits, in a petN mutant (ΔpetN) of Anabaena variabilis ATCC 29413 and that the amount of the large subunits of Cyt b6f decreased to 20-25% of that in the wild-type. The oxygen evolution activity of ΔpetN was approximately 30% of that from the wild-type, and the activity could largely be restored by the addition of N,N,N′, N′-tetramethyl-p-phenylenediamine (TMPD), which functions as an electron carrier and bypasses Cyt b6f. Both linear and cyclic electron transfer of the mutant became partially insensitive to the Cyt b6f inhibitor 2,5-dibromo-3-methyl-6-isopropylbenzoquinone (DBMIB). Although the plastoquinone (PQ) pool was largely reduced in ΔpetN under normal light conditions, the mutant had a substantially higher PSII/PSI ratio than the wild-type. State transitions in ΔpetN were abolished, as revealed by 77K fluorescence spectra and room temperature fluorescence kinetics in the presence of TMPD. Our findings strongly suggest that Cyt b6f is required for state transitions in the cyanobacteria.