Molecular Clocks in Development Life15

news and views feature mutation, tau, in the hamster. Animals with lator10. Transcription to produce frq messen- could be involved in transcriptional activa- one copy of the mutated gene show a 22-hour ger RNA is central to the clock, with levels of tion (Table 1). period in locomotor activity, and animals the frq transcript peaking in the morning. If with two mutated copies have a 20-hour peri- an inducible frq gene is expressed at the Association of clock molecules odicity8. Unfortunately, further molecular wrong time, the conidiation rhythm is In Drosophila, at least two genes are involved analysis of the tau locus has not been easy, abolished; conversely, suppression of the in clock function, and the dynamic associa- owing to the lack of well-developed genetic inducible gene resets the conidiation cycle. tion between their products ensures correct maps in hamsters. The frq messenger RNA transcript can be circadian rhythmicity. One of these, period The mouse, however, is genetically well translated into protein from either of two (per), was the first clock gene ever to 13 characterized, and a number of genes have initiation sequences (codons), resulting in be isolated . Nonsense mutations in p8er been identified that are rapidly induced by the production of two Frq polypeptides11. (which lead to an abnormally shortened Per light in its SCN9. All of these genes encode Changes in temperature regulate the choice polypeptide) result in the loss of rhythmic transcription factors that contain structural of codon used, setting the physiological tem- locomotor activity in Drosophila. Long- motifs such as the leucine zipper, which perature limits for rhythmicity. Immediately lasting phase shifts in locomotor activity are mediates protein–protein interactions, and after synthesis, phosphate groups are added also observed when pulses of heat are given the zinc finger, which mediates protein–DNA to both proteins (phosphorylation), yielding to transgenic flies bearing a heat-inducible interactions. Expression of some of these several forms of Frq12. It is not clear how Frq copy of per. Missense mutations, which genes oscillates during the light–dark cycle2, contributes to the generation of circadian cause single amino-acid substitutions in Per, but it is difficult to work out where these rhythmicity, but it is probably at the level of change the length of the circadian cycle2,14. genes fit into the clock mechanism — transcription — indeed, Frq needs to be in Cyclical expression of per is regulated by because a clock can both measure and show the nucleus (where transcription occurs) to transcription. The promoter region of the time, those genes that are directly implicated be active10. Moreover, although Frq does not per gene, which is required for initiation of in the clockwork need to be distinguished belong to any well-defined class of transcrip- transcription, is sufficient to confer oscillat- from those that mediate output signals. tion factor, it has several features that are typ- ing transcriptional regulation on another In Neurospora, the endogenous clock is ical of such factors: these include a putative gene14. But production of a stable messenger responsible for the precise circadian syn- helix–turn–helix domain through which it RNA transcript would lead to the accumula- chronization of cycles of asexual sporulation can bind DNA; a signal that allows it to be tion of this per messenger RNA and, hence, (conidiation), and the frequency (frq) gene targeted to the nucleus (a nuclear-localiza- only minor oscillations. The trick that the seems to be a central component of the oscil- tion signal); and highly charged regions that per gene uses to ensure a strict rhythm is to have a relatively short messenger RNA half- Molecular clocks in development life15. This is likely to be a very efficient regu- latory mechanism, probably involving the rhythmic activation of factors involved in During the formation of oligodendrocytes, a type linked to the formation of RNA processing. A mechanism of this kind an embryo, the events of non-neuronal cell somites. Hairy is exists in the dinoflagellate Gonyaulax poly- that lead to found in the vertebrate expressed in cyclic edra, for example — circadian expression of determination of the nervous system. When pulses with a periodicity a gene correlates with the cyclic binding of a various cell types seem placed individually in of 90 minutes — exactly protein to the 3፱ end of the messenger RNA, to be controlled by single wells, these the time that it takes to regulating the stability of the transcript16. developmental timers precursors divide a form one somite. The second fly clock gene is timeless which operate within number of times before Movement of the pulses (tim). Mutations in tim have dramatic con- individual cells. A differentiating. Two does not depend on cell sequences on per (Fig. 2): the oscillatory remarkable example of daughter cells from the displacement or on expression is lost; phosphorylation of the this timekeeping is same precursor undergo propagation of an Per protein is disrupted; and time- provided by the transition a synchronized number activating signal, and dependent nuclear transport is abolished. from rapid and of divisions, and each cell seems to have Conversely, cyclic expression of timis altered symmetrical cell differentiate at the same its own functional clock35. in per mutants17. The Tim protein seems to divisions to slow and time34. Because of its intrinsic regulate Per by interacting with it to form a asymmetrical divisions in Developmental clocks free-running property, the Per–Tim dimer. Per contains a region called embryos of the toad such as these are cell division cycle must the PAS domain (Table 1), which is necessary Xenopus laevis. This probably distinct from also be considered as a for protein–protein interactions18, and is transition always occurs those that govern molecular clock. Most now thought to be the ‘signature’ of this class at the twelfth cleavage circadian rhythms, eukaryotic cells in culture of clock molecule. PAS domains were named after fertilization, when although the molecular undergo mitosis (nuclear after the three proteins in which this struc- the embryo consists of mechanisms that control division) with a tural motif was first identified: Drosophila just over 2,000 cells proliferation and periodicity of roughly 24 Per, the mammalian Arnt, and Sim, which is known as blastomeres. differentiation hours. Is this just the product of the fly single-minded gene. The timing of the programmes are coincidence, or were These domains have since been found in transition is controlled by obviously intermingled cells sensitive to several transcription factors, where they are a clock that is intrinsic to with cellular oscillatory light–dark cycles millions thought to confer target-gene specificity, each blastomere — timing functions. For example, of years ago? If they and they are often coupled to a DNA- does not depend on expression of the avian were, what we study binding domain (a basic helix–loop–helix cell–cell interactions33. equivalent of the today as the cell cycle or bHLH domain). Drosophila Per does Another example comes Drosophila gene hairy in could represent a not contain a bHLH domain, however, from precursor cells that the chick is controlled by vestigial circadian suggesting that it may regulate transcription differentiate to form a molecular clock that is rhythm. P.S.-C. without binding DNA. Surprisingly, Tim does not seem to con-