Evidence Supporting the Hydrophobic-Mismatch Model for Cytochrome b6f-Driven State Transitions in the Cyanobacterium Synechocystis Species PCC 6803

While there is a consensus that the cytochrome <i>b</i>₆<i>f</i> complex (cyt<i>b</i>₆<i>f</i>) in algae and plants is involved in the regulatory mechanism of oxygenic photosynthesis known as light-induced state transitions (STs), no such consensus exists for cyanobacteria. Here, we provide the first direct functional evidence for cyt<i>b</i>₆<i>f</i> using single-point mutation data. We introduced a PetD-Phe124Ala substitution in the cyanobacterium <i>Synechocystis</i> sp. PCC 6803 to test the key predictions of the hydrophobic-mismatch (HMM) model for cyt<i>b</i>₆<i>f</i>-driven STs in all oxygenic photosynthetic species. These predictions concern the role of the Phe/Tyr124<i>^fg</i>^-loop-PetD and the extent and kinetic characteristics of STs. The effects of PetD-F124A mutation on STs were monitored using 77K and Pulse-Amplitude-Modulated (PAM) fluorescence. For comparison, we employed a phycobilisome (PBS)-less <i>Synechocystis</i> mutant and wild-type (WT) strain, as well as the <i>stn7</i> mutant and WT of <i>Arabidopsis</i> plant. The PetD-F124A mutation reduced the extent of STs and selectively affected the two-exponential kinetics components of the transitions. Under State 1 conditions, the mutant exhibited ~60% less energetic decoupling of PBS from photosystem I (PSI) compared to the WT. It is explainable by the HMM model with the inability of the PetD-F124A mutant, during the induction phase of the State 2→State 1 transition to adopt the cyt<i>b</i>₆<i>f</i> conformation with minimal hydrophobic thickness. PAM-derived parameters indicated that PSII electron transport function is not inhibited, and no detectable effect on cyclic electron transport around PSI was observed under low-light conditions. Circular dichroism and differential scanning calorimetry confirmed that both the PSI trimer/monomer ratio and the structural integrity of the PBSs are preserved in the mutant. The compensatory response to the mutation includes decreased PSI content and an increase in PBS rod size. In conclusion, (1) cyt<i>b</i>₆<i>f</i> is involved in cyanobacterial STs; (2) evidence is provided supporting the HMM model; (3) the electron transfer and signal transduction functions of cyt<i>b</i>₆<i>f</i> are separated into distinct domains; and (4) the signaling pathway regulating STs and pigment-protein composition in <i>Synechocystis</i> involves PetD-Phe124.