State transition in photosynthesis is a short-term balancing mechanism of energy distribution between photosystem I (PSI) and photosystem II (PSII). When PSII is preferentially excited (state 2), a pool of mobile light-harvesting complex II (LHCII) antenna proteins is thought to migrate from PSII to PSI, but biochemical evidence for a physical association between LHCII proteins and PSI in state 2 is weak. Here, using the green alga Chlamydomonas reinhardtii, which has a high capacity for state transitions, we report the isolation of PSI-light-harvesting complex I (LHCI) supercomplexes from cells locked into state 1 and state 2. We solubilized the thylakoid membranes with a mild detergent, separated the proteins by sucrose density gradient centrifugation, and subjected gradient fractions to gel-filtration chromatography. Three LHCII polypeptides were associated with a PSI-LHCI supercomplex only in state 2; we identified them as two minor monomeric LHCII proteins (CP26 and CP29) and one previously unreported major LHCII protein type II, or LhcbM5. These three LHCII proteins, in addition to the major trimeric LHCII proteins, were phosphorylated upon transition to state 2. The corresponding phylogenetic tree indicates that among the LHCII proteins associated with PSII, these three LHCII proteins are the most similar to the LHC proteins for PSI (LHCI). Our results are important because CP26, CP29, and LhcbM5, which have been viewed as belonging solely to the PSII complex, are now postulated to shuttle between PSI and PSII during state transitions, thereby acting as docking sites for the trimeric LHCII proteins in both PSI and PSII.photoacclimation ͉ photosynthesis ͉ photosystem I n oxygen-evolving photosynthetic organisms, two types of photosystems, photosystem I (PSI) and photosystem II (PSII), operate in series in the transfer of electrons from water to NADP ϩ . The two photosystems contain their own reaction centers and light-harvesting antenna systems. The PSI lightharvesting antenna system contains light-harvesting complex I (LHCI) proteins as peripheral monomeric antennas, whereas the PSII light-harvesting antenna system contains major and minor light-harvesting complex II (LHCII) proteins as peripheral trimeric and inner monomeric antennas, respectively (see review in ref. 1). The distribution of absorbed light energy between the two photosystems is dynamically balanced, ensuring maximum efficiency for photosynthetic electron transport in changing light environments (2, 3). That balance is regulated by a process termed ''state transitions'': state 1 is induced by preferential excitation of PSI, and state 2 is induced by preferential excitation of PSII (see recent reviews in refs. 4-6). This short-term adaptation process involves thylakoid-bound protein kinase(s) that is responsible for the phosphorylation of LHCII (7,8). The activation of the kinase(s) is regulated by the redox state of the intersystem pool of plastoquinone (9) through cytochrome b 6 f complexes (10). The phospho-LHCII proteins in the PSIIenri...

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Summary One of the major factors limiting biomass productivity in algae is the low thermodynamic efficiency of photosynthesis. The greatest thermodynamic inefficiencies in photosynthesis occur during the conversion of light into chemical energy. At full sunlight the light‐harvesting antenna captures photons at a rate nearly 10 times faster than the rate‐limiting step in photosynthetic electron transport. Excess captured energy is dissipated by non‐productive pathways including the production of reactive oxygen species. Substantial improvements in photosynthetic efficiency have been achieved by reducing the optical cross‐section of the light‐harvesting antenna by selectively reducing chlorophyll b levels and peripheral light‐harvesting complex subunits. Smaller light‐harvesting antenna, however, may not exhibit optimal photosynthetic performance in low or fluctuating light environments. We describe a translational control system to dynamically adjust light‐harvesting antenna sizes for enhanced photosynthetic performance. By expressing a chlorophyllide a oxygenase (CAO) gene having a 5′ mRNA extension encoding a Nab1 translational repressor binding site in a CAO knockout line it was possible to continuously alter chlorophyll b levels and correspondingly light‐harvesting antenna sizes by light‐activated Nab1 repression of CAO expression as a function of growth light intensity. Significantly, algae having light‐regulated antenna sizes had substantially higher photosynthetic rates and two‐fold greater biomass productivity than the parental wild‐type strains as well as near wild‐type ability to carry out state transitions and non‐photochemical quenching. These results have broad implications for enhanced algae and plant biomass productivity.

Photosynthetic algae cope with suboptimal levels of light and CO2. In low CO2 and excess light, the green alga Chlamydomonas reinhardtii activates a CO2 Concentrating Mechanism (CCM) and photoprotection; the latter is mediated by LHCSR1/3 and PSBS. How light and CO2 signals converge to regulate photoprotective responses remains unclear. Here we show that excess light activates expression of photoprotective and CCM-related genes and that depletion of CO2 drives these responses, even in total darkness. High CO2 levels, derived from respiration or impaired photosynthetic fixation, repress LHCSR3 and CCM genes while stabilizing the LHCSR1 protein. We also show that CIA5, which controls CCM genes, is a major regulator of photoprotection, elevating LHCSR3 and PSBS transcript accumulation while inhibiting LHCSR1 accumulation. Our work emphasizes the importance of CO2 in regulating photoprotection and the CCM, demonstrating that the impact of light on photoprotection is often indirect and reflects intracellular CO2 levels.

In green algae and plants, state transitions serve as a short-term light acclimation process to regulate light harvesting capacity of photosystems I and II (PSI and PSII). During the process, a portion of the light-harvesting complexes II (LHCII) are phosphorylated, dissociate from PSII and bind PSI to form PSI-LHCI-LHCII supercomplex. Here we report high-resolution structures of PSI-LHCI-LHCII supercomplex from Chlamydomonas reinhardtii, revealing the mechanism of assembly between PSI-LHCI complex and two phosphorylated LHCII trimers containing all four types of LhcbM proteins. Two specific LhcbM isoforms, namely LhcbM1 and LhcbM5, directly interact with the PSI core through their phosphorylated amino-terminal regions. Furthermore, biochemical and functional studies on mutant strains lacking either LhcbM1 or LhcbM5 indicate that only LhcbM5 is indispensable in the supercomplex formation. The results unraveled the specific interactions and potential excitation energy transfer routes between green algal PSI and two phosphorylated LHCIIs.

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