TIMELESS

Timeless (tim) is a gene in multiple species but is most notable for its role in Drosophila for encoding TIM, an essential protein that regulates circadian rhythm. Timeless mRNA and protein oscillate rhythmically with time as part of a transcription-translation negative feedback loop involving the period (per) gene and its protein. Discovery In 1994, timeless was discovered through forward genetic screening performed by Jeffery L. Price while working in the lab of Michael W. Young. This gene was found when they noticed an arrhythmic tim01 mutant via a P element screen. The tim01 mutation caused arrhythmic behavior, defined by the lack of ability to establish proper circadian rhythms. In 1995, the timeless gene was cloned by Amita Sehgal and partners in the lab of Michael W. Young. Unlike the Drosophila timeless gene, homologs have been discovered in other species that are non-essential for circadian rhythm. The discovery of timeless followed the discovery of the period mutants in 1971 through forward genetic screening, the cloning of per in 1984, and an experiment determining that per is circadian in 1990. This occurred during a period of rapid expansion in the field of chronobiology in the 1990s. Structure The length of the coding region of the Drosophila timeless gene is 4029 base pairs, from which a 1398 amino acid protein is transcribed. The gene starts at a consensus cap site upstream of a methionine codon. It contains 11 exons and 10 introns. In various Drosophila species, the timeless protein TIM contains more highly conserved functional domains and amino acid sequence than its counterpart, PER (protein encoded by per). CLD was the least conserved of these regions between D. virilis and D. melanogaster. These conserved parts include: the PER interaction domain, the nuclear localization signal (NLS), cytoplasmic localization domain (CLD), N-terminal end (non-functional), and C-terminal end. TIM is also known to have a basic region, which interacts with the PAS domain of the PER protein, and a central acidic region. There is also a region of unknown function near the N-terminus of the TIM protein that contains a 32 amino acid sequence that, when deleted, causes arrhythmic behavior in the fly. In various species of Drosophila, such as D. virilis and D. melanogaster, the initiating methionine for translation of the timeless gene into TIM is in different places, with the D. virilis start site downstream of the start site in D. melanogaster.