Regulatory Study On Molecular Mechanism Of State Transition In Cyanobacteria

Cyanobacteria are prokaryotes capable of oxygenic photosynthesis, and are one of the most popular model organisms for photosynthesis research. In the process of evolution of oxygen-evolution organisms, an important regulatory mechanism, state transition, is generated, and it can help cyanobacterial cells to adapt changes of rapid evironmetal conditions, including spectral quality, and to balance activities of the 2 photosytems (PSs). Presently, it has been widely proposed that in cyanobacteria, state transitions are triggered by changes in the redox state of electron carriers, such as plastoquinone (PQ) pool and cytochrome b6f (Cytb6f) complex. However, it is unclear whether other photosynthetic proteins between the 2 PSs are also involved in this regulatory process. In addition, the biochemical evidence for interaction between phycobilisome (PBS) and PSs is weak. In this dissertation, thus, the 2 questions are to be further investigation, and the related results are listed as follows:First, the mobile fraction of PBS was found to be maximum at a particular redox value of QA (i.e., 0.52). An upward or downward shift in the redox value leads to a decrease in this mobile fraction of PBS. Furthermore, the regularoy effect of the redox state of QA on PBS mobility was found to be independent of the effect exerted by the plastoquinone (PQ) pool. These findings indicate for the first time that PBS mobility is regulated by the QA redox state in cyanobacteria. Moreover, this was an important revision for control of the PQ pool redox state on cyanobacterial state transitions.Secondly, the biochemical changes in associating populations of PBS with photosystems II (PSII) and I (PSI), caused by light-induced and redox-induced state 1– state 2 transitions was first identified in a cyanobacterium. Associating populations of the PBS rod with the PSI trimer were more prevalent in state 2 than in state 1 compared with the PSII dimer, regardless of light-induced or redox-induced state transitions. Simultaneous association of the PBS core with the PSII dimer and PSI monomer was specific to state 2, and the PSII dimer and PSI trimer principally appeared in state 1 during the redox-induced state transitions. The simultaneous associations were not observed during light-induced state transitions. Furthermore, the changes in the association of populations of the PBS core and the PBS rod with the PSI trimer were opposed during the redox-induced state transition. Analysis of these changes in associating populations not only clarified the current controversial routes of energy transfer of cyanobacterial state transition at the biochemical level, but also permitted the proposal of a novel route of"energy spillover"in cyanobacteria.In conclusion, the current findings not only improved the response mechanism of cyanobacterial state transition, but also confirmed its various routes of energy transfer at the biochemical level and proposed a novel route of energy spillover during state transition in cyanobacteria.