plants has completely different components to those in the animal, fungus, or bacterial clocks. The plant clock does have a conceptual similarity to the animal clock in that it consists of a series of interlocking transcriptional feedback loops. The genes involved in the clock show their peak expression at a fixed time of day. The first genes identified in the plant clock were TOC1, CCA1 and LHY. The peak expression of the CCA1 and LHY genes occurs at dawn, and the peak expression of the TOC1 gene occurs roughly at dusk. CCA1/LHY and TOC1 proteins repress the expression of each other's genes. The result is that as CCA1/LHY protein levels start to reduce after dawn, it releases the repression on the TOC1 gene, allowing TOC1 expression and TOC1 protein levels to increase. As TOC1 protein levels increase, it further suppresses the expression of the CCA1 and LHY genes. The opposite of this sequence occurs overnight to re-establish the peak expression of CCA1 and LHY genes at dawn. There is much more complexity built into the clock, with multiple loops involving the PRR genes, the Evening Complex and the light sensitive GIGANTIA and ZEITLUPE proteins. Bacterial clocks In bacterial circadian rhythms, the oscillations of the phosphorylation of cyanobacterial Kai C protein was reconstituted in a cell free system (an in vitro clock) by incubating KaiC with KaiA, KaiB, and ATP. Post-transcriptional modification For a long time it was thought the transcriptional activation/repression cycles driven by the transcriptional regulators constituting the circadian clock was the main driving force for circadian gene expression in mammals. More recently, however, it was reported that only 22% of messenger RNA cycling genes are driven by de novo transcription. RNA-level post-transcriptional mechanisms driving rhythmic protein expression were later reported, such as mRNA polyadenylation dynamics. Fustin and co-workers identified methylation of internal adenosines (m6A) within mRNA (notably of clock transcripts themselves) as a key regulator of the circadian period. Inhibition of m6A methylation via pharmacological inhibition of cellular methylations or more specifically by siRNA-mediated silencing of the m6A methylase Mettl3 led to the dramatic elongation of the circadian period. In contrast, overexpression of Mettl3 in vitro led to a shorter period. These observations clearly demonstrated the importance of RNA-level post-transcriptional regulation of the circadian clock, and concurrently established the physiological role of (m6A) RNA methylation. Post-translational modification The autoregulatory feedback loops in clocks take about 24 hours to