Circadian Control of cab Gene Transcription and mRNA Accumulation in Arabidopsis

Abstract

An intriguing property of many organisms is their ability to exhibit rhythmic cellular events that continue independently of environmental stimuli. These rhythmic processes are generated by an endogenous mechanism known as the biological clock. We wished to determine whether Arabidopsis thaliana will serve as a model plant system for a molecular genetic dissection of the circadian clock. To this end, we investigated the expression of Arabidopsis chlorophyll a/b-binding protein (cab) genes throughout the circadian cycle. Steady-state mRNA levels of the cab2 and cab3 genes showed a dramatic circadian cycling in plants shifted from light/dark cycles to constant darkness, whereas the cabl mRNA level exhibited little or no cycling under the same conditions. Analysis of cab promoter fusions in transgenic tobacco revealed that both the cabl and cab2 5' upstream regions confer circadian-regulated expression on a chloramphenicol acetyltransferase (cat) reporter gene. In vitro nuclear run-on transcription assays also indicated that the transcription of the cabl and cab2 genes is circadian regulated in Arabidopsis. Taken together, these data suggest that a post-transcriptional mechanism influences cabl mRNA levels in Arabidopsis. The identification of circadian-regulated cis-acting elements in the cabl and cab2 upstream regions will provide powerful tools for both molecular and genetic analysis of the higher plant circadian clock.

Progression and Specificity of Protein Oxidation in the Life Cycle of Arabidopsis thaliana

“…In the case of higher plants, several reports have shown that the mRNA levels of cab transcripts fluctuate diurnally (reviewed in Ref. 28) with cab mRNA levels peaking in the late morning and reaching a minimum in the late evening. Interestingly, the carbonylation of this protein also fluctuated such that oxidation was mainly observed during the night (Table I).…”

Expression of tobacco genes for light-harvesting chlorophyll a/b binding proteins of photosystem II is controlled by two circadian oscillators in a developmentally regulated fashion.

“…It has been shown recently that the phytochrome-coupled circadian clock regulates the expression of Cab genes at the level of transcription (18,19). Cis-regulatory elements and transcnption factors mediating clock-responsive gene expression were recently identified (17,29).…”

“…The phytochrome-and circadian-clock-modulated expression of various Cab genes (encoding the chlorophyll a/b binding proteins of the light-harvesting complexes of photosystems I and II) has been much studied (15,16). Recent results clearly suggested that (i) phytochrome and the circadian clock control the expression of the Cab genes in seedlings and mature plants, at least partially at the level of transcription (17)(18)(19), and that (ii) the expression of the Cab genes and the setting or synchronization of the circadian oscillator(s) is regulated by the very low fluence response of phytochrome in etiolated wheat seedlings (20). In etiolated Arabidopsis thaliana seedlings, the phytochrome-regulated expression of Cab genes is preceded by a period during which their expression is controlled by a light-independent developmental program (21).…”

Different circadian oscillators control Ca 2+ fluxes and Lhcb gene expression

“…This [Ca 2ϩ ] c oscillation might play a role in the central oscillator mechanism, the phototransduction path, or in controlling circadian outputs. To test whether the [Ca 2ϩ ] c oscillation might control outputs, we focused on a well characterized circadian rhythm, namely, the rate of transcription of the light-harvesting complex (Lhc)b gene family (7)(8)(9)(10), for which there is evidence of [Ca 2ϩ ] c control of transcription (11,12). Transgenic plants have been developed by using a luciferase reporter fused to the promoter of one of the Lhcb genes (Lhcb::Luc) that enables continuous precise recording of the promoter activity (7,8,10).…”

“…To test whether the [Ca 2ϩ ] c oscillation might control outputs, we focused on a well characterized circadian rhythm, namely, the rate of transcription of the light-harvesting complex (Lhc)b gene family (7)(8)(9)(10), for which there is evidence of [Ca 2ϩ ] c control of transcription (11,12). Transgenic plants have been developed by using a luciferase reporter fused to the promoter of one of the Lhcb genes (Lhcb::Luc) that enables continuous precise recording of the promoter activity (7,8,10). A series of experiments involving microinjection of calcium and calmodulin into tomato cells has led to a model for [Ca 2ϩ ] c control of Lhc promoters (11,12 To germinate seedlings, seeds were rinsed in 70% ethanol, sterilized in 20% Clorox (5.25% sodium hypochlorite) for 15 min, and washed in sterilized water three to four times.…”