Regulation of electron transport at photosystem II

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We examined the two major mechanisms by which balance of photosystem I (PSI) and Photosystem II (PSII) in the photosynthetic electron transport in higher plants was regulated in response to the variation of light environment, focusing on the regulation at PSII.1.Regulation of electron transport at primary quinone acceptor (QA) L protein which is a component of PSII complex and encoded by chloroplast psbL gene was found to be involved in the regulation of electron transport at QA site by in vitro reconstitution experiments. Plastoquinone-9 (PQ-9) depleted PSII reaction center core complex, consisting of CP47/D1/D2/Cytb-559/I proteins was isolated and submitted to the reconstitution with PQ-9. Only when the reconstitution was carried out with the L protein, PQ-9 reinserted in the complex and worked as QA in the resulting complex. Recombinant L protein was successfully over expressed in E.coli.and found to stand for the isolated L protein from PSII particles. Based on these findings, the … More molecular mechanism of the regulation of electron transport at QA site is being examined by the in vitro reconstitution with various types of mutant L protein.2.Regulation of the gene expression of psbC and psbD by light at transcription level There are two light-responsive promoters in chloroplast psbD/C gene cluster which encodes several important proteins of PSII.One (D/C-3 promoter) produces mRNA for D2 protein and the other (D/C-4 promoter) does that for CP43 protein. Upon light illumination, the former is reversibly activated while the latter is inactivated, respectively, suggesting that synthesis of these two proteins responses to light intensity through the regulation of activities of these promoters. We succeeded in the patrial purification of a nuclear-encoded regulatory factor which specifically activates the D/C-3 promoter. This factor seems to be a sequence specific DNA binding protein which recognizes the cis-element residing upstream region of the D/C-3 transcription initiation site. Less