The Circadian Rhythm in the Expression of Nitrate Reductase in Tomato is Driven by Changes in Protein Level and Not by Protein Phosphorylation

Abstract

The presence of clock-controlled processes in virtually all living organisms is suggestive of a fundamental biological relevance. Circadian rhythms are involved in diverse processes ranging from daily sleep/wake cycles in humans, to gene transcription in prokaryotes, as well as leaf movements, stomatal, and photosynthesis in higher plants. Attempts to understand both the mechanism and purposes of circadian oscillations in higher plants have revealed numerous genes that exhibit circadian regulation of transcription. By comparison, there are relatively few examples of circadian rhythms in enzyme activity. One reason is that the protein products are often abundant and quite stable, so that a circadian oscillation in gene transcription and subsequent translation often does not result in any detectable oscillation at the protein level. This, for example, is the case for the chlorophyll a/b binding protein and rubisco activase (1). However, because circadian rhythms are observed in numberous whole plant processes it is apparent that many enzymes must have circadian rhythms in activity. Circadian oscillations in enzyme activity could be conferred in various ways including cycling protein levels, changing substrate concentrations, cycling levels of allosteric effectors, or oscillations in protein phosphorylation or redox state.