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Chloroplast is a typical plant cell organelle where photosynthesis takes place. In this study, a total of 1,808 chloroplast core proteins in Arabidopsis thaliana were reliably identified by combining the results of previously published studies and our own predictions. We then constructed a chloroplast protein interaction network primarily based on these core protein interactions. The network had 22,925 protein interaction pairs which involved 2,214 proteins. A total of 160 previously uncharacterized proteins were annotated in this network. The subunits of the photosynthetic complexes were modularized, and the functional relationships among photosystem I (PSI), photosystem II (PSII), light harvesting complex of photosystem I (LHC I) and light harvesting complex of photosystem I (LHC II) could be deduced from the predicted protein interactions in this network. We further confirmed an interaction between an unknown protein AT1G52220 and a photosynthetic subunit PSI-D2 by yeast two-hybrid analysis. Our chloroplast protein interaction network should be useful for functional mining of photosynthetic proteins and investigation of chloroplast-related functions at the systems biology level in Arabidopsis.
We have identified and characterized a novel tobacco gene, called ZGT (from the Chinese phrase zhong guang tiaokong, or clock and light controlled), that is regulated by the circadian clock and light. ZGT transcripts have alternate forms that are differentially expressed in different tissues. ZGT is expressed rhythmically in light/dark cycles and in constant light. Constitutive expression of ZGT sustains the expression of the clock-controlled LHCB1*1 gene in constant darkness, when it would normally dampen, but does not affect LHCB1*1 expression in constant light. ZGT expression is induced rapidly by light, and overexpression of ZGT increases the sensitivity of the circadian oscillator to brief light pulses. The ZGT promoter includes a G-box motif that is found in many light-regulated promoters in plants and is the same as the E box described for rhythmically regulated promoters of animal circadian clock genes. The ZGT promoter also includes "evening element" motifs that are correlated with circadian control of plant genes. We postulate that light- and clock-regulated expression of ZGT acts as a coupling agent between the central circadian oscillator and rhythmic LHCB1*1 expression and that it may function as a component in plant phototransduction pathways.
Circadian biological clocks control many biological events, but the pathways by which these events are controlled are largely unknown. Based on a model suggesting that cytosolic-free calcium levels control the expression of the Lhcb gene in plants, we tested whether the circadian oscillation of free calcium is responsible for driving the rhythm of Lhcb expression. We found that these rhythms free-run with different periods in tobacco seedlings in constant conditions. Moreover, robust oscillations of Lhcb promoter activity continued in undifferentiated tobacco calli in the absence of Ca(2+) oscillations. Therefore, these two circadian rhythms are not linked hierarchically. These data provide evidence for separate circadian pacemakers controlling molecular events in plants.
The small gene family encoding the chlorophyll a/b-binding proteins of photosystem II (CABII or lhcb) is known to exhibit circadian rhythms of mRNA abundance in Chlamydomonas reinhardtii. In this study we investigated the role of transcription in the phenomenon. We used as reporters Chlamydomonas genes that encode nitrate reductase (NITI) and arylsulfatase (ARS2) transcriptionally fused to sequences upstream of one of the CABII genes (called CABII-1). We found that both reporters exhibited the same circadian rhythm of mRNA abundance in phase, period, and amplitude as does the endogenous CABII-1 gene. We also evaluated the efficacy of arylsulfatase enzymatic activity as a reporter and found that its half-life is too long to make it a useful reporter of rhythmic transcription during a circadian or diurnal cycle. The amount of mRNA synthesis from the CABII-1 gene was examined by in vivo labeling experiments and a circadian rhythm in transcription rate was demonstrated. In vivo labeling also revealed a circadian rhythm of mRNA synthesis for the CABII gene family as a whole. The results from the transcriptional reporter assays together with the in vivo labeling experiments strongly support the conclusion that the biological clock regulates the transcriptional activity of the CABII-I gene, and moreover that regulation at the transcriptional level is the predominant mode by which the clock regulates this gene.
In Chlamydomonas reinhardtii cells, the amount of mRNA coding for the chlorophyll a/b binding proteins of photosystem II (cab II) oscillates in light/dark cycles and in constant dim light. This rhythmic behavior applies to the overall expression of the entire cab II gene family as well as to a single member of the family. The highest mRNA abundance is found in the middle of the subjective day and the lowest in the middle of the subjective night. In constant darkness the cab II mRNA rhythm damps rapidly. The cab II mRNA rhythm persists in non-growing cells under CO2-starvation conditions indicating that the cab II mRNA rhythm is not merely a consequence of cell division, although cell division may influence the amplitude of the cab II mRNA rhythm. The properties of this mRNA oscillation conform to all the major characteristics of circadian rhythms: the period in constant conditions is about 24 h, the rhythm entrains to 24 h light/dark cycles, and the period is temperature compensated. This report is the first demonstration of a circadian rhythm of cab II gene expression in single cells. beta-Tubulin mRNA also shows an oscillation in its abundance in LD cycles and in constant dim light, although its peak-to-trough amplitude is smaller than that of the cab II mRNA rhythm. The beta-tubulin mRNA rhythm peaks in the early night in LD cycles, but in constant illumination, it peaks at about the same circadian phase (i.e., mid-subjective day) as does the cab II mRNA rhythm. Finally, the amount of mRNA coding for mitochondrial cytochrome c is rhythmic in a light/dark cycle but is constant in constant dim light or constant darkness. Surprisingly, this mRNA exhibits a daily oscillation in constant dim light under the specific condition of CO2-depletion.