Figure 15–67 The core circadian oscillator of cyanobacteria. (A) KaiC is a combined kinase and phosphatase that phosphorylates and dephosphorylates itself on two adjacent sites. In the absence of other proteins, the phosphatase activity is dominant, and the protein is mostly unphosphorylated. The binding of KaiA to KaiC suppresses the phosphatase activity and promotes the kinase activity, leading to KaiC phosphorylation, first at site 1 and then at site 2, resulting in diphosphorylated KaiC. KaiC then dephosphorylates itself slowly at site 1, even in the presence of KaiA, so that KaiC is phosphorylated only at site 2. This form of KaiC interacts with KaiB, which blocks the stimulatory effects of KaiA, thereby reducing the rate of KaiC phosphorylation and allowing dephosphorylation to occur. Diphosphorylated KaiC increases in abundance during the day and peaks around dusk. It activates other proteins that phosphorylate a transcription regulator (RpaA), which then stimulates expression of some genes (the dusk genes that peak in early evening) and inhibits expression of other genes (the dawn genes that peak in the morning). When KaiC dephosphorylation gradually occurs during the night, these effects are reversed: dusk genes are turned off and dawn genes are turned on.
(B) In this experiment, the three Kai proteins were purified and mixed in a test tube with ATP (which is required for KaiC kinase activity). Every two hours over the next 3 days, the KaiC protein was analyzed by polyacrylamide gel electrophoresis, in which the phosphorylated form of KaiC migrates more slowly (upper band, P-KaiC) than the nonphosphorylated form (lower band, NP-KaiC). The three different phosphorylated forms of KaiC are not distinguished by this method. The phosphorylation of KaiC oscillates with a roughly 24-hour period. (C) The amount of phosphorylated and unphosphorylated KaiC in the experiment in B is plotted on this graph, along with the amount of total protein. (B and C, from M. Nakajima et al., Science 308:414–415, 2005. With permission from AAAS.) the transcription of its target genes. NFκB also activates the transcription of the gene that encodes IκBα, creating a negative feedback loop, which can produce prolonged oscillations in NFκB activity with sustained extracellular signaling.
Some small, hydrophobic signal molecules, including steroid and thyroid hormones, diffuse across the plasma membrane of the target cell and activate intracellular receptor proteins that directly regulate the transcription of specific genes.
In many cell types, gene expression is governed by circadian clocks, in which delayed negative feedback produces 24-hour oscillations in the activities of transcription regulators, anticipating the cell's changing needs during the day and night.
In plants, as in animals, cells are in constant communication with one another. Plant cells communicate to coordinate their activities in response to the changing conditions of light, dark, and temperature, which guide the plant’s cycle of growth, flowering, and fruiting. Plant cells also communicate to coordinate activities in their roots, stems, and leaves. In this final section, we consider how plant cells signal to one another and how they respond to light. Less is known about the receptors and intracellular signaling mechanisms involved in cell communication in plants than is known in animals, and we will concentrate mainly on how the receptors and intracellular signaling mechanisms differ from those used by animals.