Transcription-translation feedback loop (TTFL) is a cellular model for explaining circadian rhythms in behavior and physiology. Widely conserved across species, the TTFL is auto-regulatory, in which transcription of clock genes is regulated by their own protein products.

Circadian rhythms have been documented for centuries. For example, French astronomer Jean-Jacques d'Ortous de Mairan noted the periodic 24-hour movement of Mimosa plant leaves as early as 1729. However, science has only recently begun to uncover the cellular mechanisms responsible for driving observed circadian rhythms. The cellular basis of circadian rhythms is supported by the fact that rhythms have been observed in single-celled organisms.

Beginning in the 1970s, experiments conducted by Ron Konopka and colleagues, in which forward genetic methods were used to induce mutation, revealed that Drosophila melanogaster specimens with altered period (Per) genes also demonstrated altered periodicity. As genetic and molecular biology experimental tools improved, researchers further identified genes involved in sustaining normal rhythmic behavior, giving rise to the concept that internal rhythms are modified by a small subset of core clock genes. Hardin and colleagues (1990) were the first to propose that the mechanism driving these rhythms was a negative feedback loop. Subsequent major discoveries confirmed this model; notably experiments led by Thomas K. Darlington and Nicholas Gekakis in the late 1990s that identified clock proteins and characterized their methods in Drosophila and mice, respectively. These experiments gave rise to the transcription-translation feedback loop (TTFL) model that has now become the dominant paradigm for explaining circadian behavior in a wide array of species.

The TTFL is a negative feedback loop, in which clock genes are regulated by their protein products. Generally, the TTFL involves two main arms: positive regulatory elements that promote transcription and protein products that suppress transcription. When a positive regulatory element binds to a clock gene promoter, transcription proceeds, resulting in the creation of an mRNA transcript, and then translation proceeds, resulting in a protein product. There are characteristic delays between mRNA transcript accumulation, protein accumulation, and gene suppression due to translation dynamics, post-translational protein modification, protein dimerization, and intracellular travel to the nucleus. Across species, proteins involved in the TTFL contain common structural motifs such PAS domains, involved in protein-protein interactions, and bHLH domains, involved in DNA binding.

Once enough modified protein products accumulate in the cytoplasm, they are transported into the nucleus where they inhibit the positive element from the promoter to stop transcription of clock genes. The clock gene is thus transcribed at low levels until its protein products are degraded, allowing for positive regulatory elements to bind to the promoter and restart transcription. The negative feedback loop of the TTFL has multiple properties important for the cellular circadian clock. First, it results in daily rhythms in both gene transcription and protein abundance and size, caused by the delay between translation and negative regulation of the gene. The cycle's period, or time required to complete one cycle, remains consistent in each individual and, barring mutation, is typically near 24 hours. This enables stable entrainment to the 24 hour light-dark cycle that Earth experiences. Additionally, the protein products of clock genes control downstream genes that are not part of the feedback loop, allowing clock genes to create daily rhythms in other processes, such as metabolism, within the organism. Lastly, the TTFL is a limit cycle, meaning that it is a closed loop that will return to its fixed trajectory even if it is disturbed, maintaining the oscillatory path on its fixed 24-hour period.

The presence of the TTFL is highly conserved across animal species; however, many of the players involved in the process have changed across evolutionary time within different species. There are differences in the genes and proteins involved in the TTFL when comparing plants, animals, fungi and other eukaryotes. This suggests that a clock that follows the TTFL model has evolved multiple times during the existence of life.

The TTFL was first discovered in Drosophila, and the system shares several components with the mammalian TTFL. Transcription of the clock genes, Period (per) and Timeless (tim), is initiated when positive elements Cycle (dCYC) and Clock (dCLK) form a heterodimer and bind E-box promoters, initiating transcription. During the day TIM is degraded; light exposure facilitates CRY binging to TIM, which leads to TIM's ubiquitination and eventual degradation. During the night, TIM and PER are able to form heterodimers and accumulate slowly in the cytoplasm, where PER is phosphorylated by the kinase DOUBLETIME (DBT). The post-transcriptional modification of multiple phosphate groups both targets the complex for degradation and facilitates nuclear localization. In the nucleus, the PER-TIM dimer binds to the CYC-CLK dimer, which makes the CYC-CLK dimer release from the E-boxes and inhibits transcription. Once PER and TIM degrade, CYC-CLK dimers are able to bind the E-boxes again to initiate transcription, closing the negative feedback loop.

Secondary feedback loops interact with this primary feedback loop. CLOCKWORK ORANGE (CWO) binds the E-boxes to act as a direct competitor of CYC-CLK, therefore inhibiting transcription. PAR-DOMAIN PROTEIN 1 ε (PDP1ε) is a feedback activator and VRILLE (VRI) is a feedback inhibitor of the Clk promoter, and their expression is activated by dCLK-dCYC. Ecdysone-induced protein 75 (E75) inhibits clk expression and tim-dependently activates per transcription. All of these secondary loops act to reinforce the primary TTFL.