Light-intensity-dependent expression of Lhc gene family encoding light-harvesting chlorophyll-a/b proteins of photosystem II in Chlamydomonas reinhardtii

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.     

Promoters from genes for plastid proteins possess regions with different sensitivities toward red and blue light.

The light-regulated expression of eight nuclear-encoded genes for plastid proteins from spinach (Spinacia oleracea) (RBCS-1 and CAB-1; ATPC and ATPD, encoding the subunits gamma and delta of the ATP synthase; PC and FNR; PSAD and PSAF, encoding the subunits II and III of photosystem I reaction center) was analyzed with promoter/beta-glucuronidase (GUS) gene fusions in transgenic tobacco (Nicotiana tabacum and Nicotiana plumbaginifolia) seedlings and mature plants under standardized light and growth conditions. Unique response patterns were found for each of these promoters. GUS activities differed more than 30-fold. Strong promoters were found for the PC and PSAD genes. On the other hand, the ATPC promoter was relatively weak. Expression of the CAB/GUS gene fusion in etiolated material was at the detection limit; all other chimeric genes were expressed in the dark as well. Light stimulation of GUS activities ranged from 3- (FNR promoter) to more than 100-fold (CAB-1 promoter). The FNR promoter responded only to red light (RL) and not significantly to blue light (BL), whereas the PC promoter contained regions with different sensitivities toward RL and BL. Furthermore, different RNA accumulation kinetics were observed for the PSAF, CAB, FNR, and PC promoter/GUS gene fusions during de-etiolation, which, at least in the case of the PSAF gene, differed from the regulation of the corresponding endogenous genes in spinach and tobacco. The results suggest either that not all cis elements determining light-regulated and quantitative expression are present on the spinach promoter fragments used or that the spinach cis-regulatory elements respond differently to the host (tobacco) regulatory pathway(s). Furthermore, as in tobacco, but not in spinach, the trans-gene hardly responds to single light pulses that operate through phytochrome. Taken together, the results suggest that the genes have been independently translocated from the organelle to the nucleus during phylogeny. Furthermore, each gene seems to have acquired a unique set of regulatory elements.     

Immuno-electron microscopic quantification of the fucoxanthin chlorophyll a/c binding polypeptides Fcp2, Fcp4, and Fcp6 of Cyclotella cryptica grown under low- and high-light intensities.

The diatom Cyclotella cryptica was grown under low- and high-intensity white light of 50 and 500 micromol photons m-2 s-1, respectively. Western immunoblotting showed that the diatom adapted its light-harvesting apparatus, giving rise to different amounts of distinct fucoxanthin chlorophyll a/c binding polypeptides (Fcp). The amount of Fcp2 was approximately two-fold higher under low-light than under high-light conditions, whereas the amount of Fcp6 increased four- to five-fold under high-light conditions. For Fcp4, no significant differences were detected in response to either light regime. Cells of Cyclotella grown under high- and low-light intensity were subjected to immunoelectron microscopy. Quantification of the gold label, expressed as gold particles per microm2, confirmed the results obtained by Western immunoblotting. Exposure to low light resulted in the detection of approximately six times more Fcp2-bound gold particles per microm2 in thylakoid membranes, whereas in cells grown under high light the number of Fcp6-bound gold particles increased ten-fold. For Fcp4, similar amounts of gold particles per microm2 were counted under the two light regimes. These immunocytochemical results confirmed molecular data derived from phylogenetic analyses of the sequences of genes encoding fucoxanthin chlorophyll a/c binding polypeptides (fcp genes) and from measurements of steady-state fcp mRNA concentrations. The results show that Fcp2 and Fcp6 accumulate under low- and high-light intensity, respectively, whereas Fcp4 seems to be constitutively synthesized.