Abstract
In oxygen-evolving photosynthesis, the two photosystems, photosystem I (PSI) and photosystem II (PSII), function in parallel, and their excitation levels must be balanced to maintain an optimal photosynthetic rate under various light conditions. State transitions balance excitation energy between the two photosystems by redistributing light-harvesting complex II (LHCII) proteins. Here we describe two RNA interference (RNAi) mutants of the green alga Chlamydomonas reinhardtii with one of the minor monomeric LHCII proteins, CP29 or CP26, knocked down. These two proteins have been identified in PSI-LHCI supercomplexes that harbor mobile LHCII proteins from PSII under a state where PSII is preferentially excited (State 2). We show that both the CP29 and CP26 RNAi mutants undergo reductions in the PSII antenna size during a transition from State 1 (a state where PSI is preferentially excited) to State 2, as reflected by nonphotochemical quenching of fluorescence, low temperature fluorescence spectra, and functional absorption cross-section. However, the undocked LHCIIs from PSII do not re-associate with PSI in the CP29-RNAi (b4i) mutant because the antenna size of PSI was not complementary increased. The mobile LHCIIs in the CP26-RNAi (b5i) mutant, however, re-associate with PSI, whose PSI-LHCI/II supercomplex is visualized on a sucrose density gradient. This study clarifies that CP29, not CP26, is an essential component in state transitions and demonstrates that CP29 is crucial when mobile LHCIIs re-associate with PSI under State 2 conditions.
Highlights
Mize photosynthetic electron transport efficiency and avoid damage during light stress conditions, plants and green algae control the distribution of excitation energy between the two photosystems [1, 2]
A transfer of the cells from State 1 to State 2 increases the number of phosphorylated residues in CP29 from 2 to 4, and these residues were mapped at the interface of the photosystem II (PSII) core and peripheral antenna proteins [11]
A pool of phosphorylated CP26, CP29, and light-harvesting complex II (LHCII) type I, which are dissociated from PSII during a transition to State 2 [12], was detected in the PSILHCI/II supercomplex fraction in wild type (WT) and the b5i mutant; all of the phosphorylated LHCII proteins were found in the free LHCII fraction in the b4i mutant (Fig. 5)
Summary
Mize photosynthetic electron transport efficiency and avoid damage during light stress conditions, plants and green algae control the distribution of excitation energy between the two photosystems (state transition) [1, 2]. CP29 Is Essential for Photosynthetic State Transitions methods, including phosphorylation of LHCII proteins, fluorescence quenching analysis, antennae measurements, fluorescence emission and excitation spectra, and formation of a PSILHCI/II supercomplex.
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