Excessive light conditions repressed the levels of mRNAs accumulation of multiple Lhc genes encoding light-harvesting chlorophyll-a/b (LHC) proteins of photosystem (PS)II in the unicellular green alga, Chlamydomonas reinhardtii. The light intensity required for the repression tended to decrease with lowering temperature or CO 2 concentration. The responses of six LhcII genes encoding the major LHC (LHCII) proteins and two genes (Lhcb4 and Lhcb5) encoding the minor LHC proteins of PSII (CP29 and CP26) were similar. The results indicate that the expression of these Lhc genes is coordinately repressed when the energy input through the antenna systems exceeds the requirement for CO 2 assimilation. The Lhc mRNA level repressed under high-light conditions was partially recovered by adding the electron transport inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea, suggesting that redox signaling via photosynthetic electron carriers is involved in the gene regulation. However, the mRNA level was still considerably lower under high-light than under low-light conditions even in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Repression of the Lhc genes by high light was prominent even in the mutants deficient in the reaction center(s) of PSII or both PSI and PSII. The results indicate that two alternative processes are involved in the repression of Lhc genes under high-light conditions, one of which is independent of the photosynthetic reaction centers and electron transport events.Photosynthesis is regulated at various levels in response to fluctuating light intensity under various ambient temperature and nutrient conditions. The proper responses to the various environmental cues are necessary for photosynthetic plants to use light energy efficiently and to protect themselves from photoinhibitory damage caused by excessive irradiance (Aro et al., 1993;Long et al., 1994;Osmond, 1994). Excessive light energy absorbed by chlorophyll is dissipated by non-radiative processes (Crofts and Yerkes, 1994;Horton et al., 1996;Gilmore, 1997) and is properly distributed between two photosystems (PS) by state transition (Allen, 1995;Gal et al., 1997), whereas the energy input is regulated by changes in the size of the light-harvesting antenna systems to modulate the optical cross section.Light-harvesting chlorophyll a/b (LHC)II proteins, which are major components of light-harvesting antennae of PSII in higher plants and green algae, typically change their abundance in response to the intensity of irradiance (Anderson et al., 1988(Anderson et al., , 1995. Under stress and intense light, enhanced amounts of reactive oxygen species will react with proteins and lipids, not only in chloroplasts but also in the cytosol, and will induce various types of photodamage. Therefore, the quality and quantity control of the LHC protein complex is required to avoid photodamage by alleviating excitation energy pressure. Although the LHC protein complex could be controlled by various mechanisms including pigment synthesis, the repression of the...