We have isolated three alleles of a novel Drosophila clock gene, double-time (dbt). Short- (dbtS) and long-period (dbtL) mutants alter both behavioral rhythmicity and molecular oscillations from previously identified clock genes, period and timeless. A third allele, dbtP, causes pupal lethality and eliminates circadian cycling of per and tim gene products in larvae. In dbtP mutants, PER proteins constitutively accumulate, remain hypophosphorylated, and no longer depend on TIM proteins for their accumulation. We propose that the normal function of DOUBLETIME protein is to reduce the stability and thus the level of accumulation of monomeric PER proteins. This would promote a delay between per/tim transcription and PER/TIM complex function, which is essential for molecular rhythmicity.

The cloning of double-time (dbt) is reported. DOUBLETIME protein (DBT) is most closely related to human casein kinase Iepsilon. dbtS and dbtL mutations, which alter period length of Drosophila circadian rhythms, produce single amino acid changes in conserved regions of the predicted kinase. dbtP mutants, which eliminate rhythms of per and tim expression and constitutively overproduce hypophosphorylated PER proteins, abolish most dbt expression. dbt mRNA appears to be expressed in the same cell types as are per and tim and shows no evident oscillation in wild-type heads. DBT is capable of binding to PER in vitro and in Drosophila cells, suggesting that a physical association of PER and DBT regulates PER phosphorylation and accumulation in vivo.

Eclosion, or emergence of adult flies from the pupa, and locomotor activity of adults occur rhythmically in Drosophila melanogaster, with a circadian period of about 24 hours. Here, a clock mutation, timeless (tim), is described that produces arrhythmia for both behaviors. The effects of tim on behavioral rhythms are likely to involve products of the X chromosome-linked clock gene period (per), because tim alters circadian oscillations of per RNA. Genetic mapping places tim on the left arm of the second chromosome between dumpy (dp) and decapentaplegic (dpp).

In wild-type Drosophila, the period protein (PER) is found in nuclei of the eyes and brain, and PER immunoreactivity oscillates with a circadian rhythm. The studies described here indicate that the nuclear localization of PER is blocked by timeless (tim), a second chromosome mutation that, like per null mutations, abolishes circadian rhythms. PER fusion proteins without a conserved domain (PAS) and some flanking sequences are nuclear in tim mutants. This suggests that a segment of PER inhibits nuclear localization in tim mutants. The tim gene may have a role in establishing rhythms of PER abundance and nuclear localization in wild-type flies.

The timeless mutation (tim)

A mutation (K38R) which specifically eliminates kinase activity was created in the Drosophila melanogaster ckI gene (doubletime [dbt]). In vitro, DBT protein carrying the K38R mutation (DBT K/R ) interacted with Period protein (PER) but lacked kinase activity. In cell culture and in flies, DBT K/R antagonized the phosphorylation and degradation of PER, and it damped the oscillation of PER in vivo. Overexpression of short-period, long-period, or wild-type DBT in flies produced the same circadian periods produced by the corresponding alleles of the endogenous gene. These mutations therefore dictate an altered "set point" for period length that is not altered by overexpression. Overexpression of the DBT K/R produced effects proportional to the titration of endogenous DBT, with long circadian periods at lower expression levels and arrhythmicity at higher levels. This first analysis of adult flies with a virtual lack of DBT activity demonstrates that DBT's kinase activity is necessary for normal circadian rhythms and that a general reduction of DBT kinase activity does not produce short periods.Circadian rhythms are molecular, physiological, or behavioral processes which occur with a periodic oscillation of approximately 24 h. These rhythms can be entrained by environmental cues such as light/dark cycles, but they persist in the absence of such cues, thus demonstrating the presence of an endogenous clock (reviewed in reference 40). Drosophila melanogaster has been widely used as a model organism for a genetic analysis of circadian rhythms, and many of the gene products comprising the endogenous clock mechanism were identified in this organism, including the first discovered clock gene, period (per) (reviewed in reference 42). Molecular genetic analysis of these rhythms has elucidated a basic clock mechanism consisting of oscillating clock gene products, which regulate their own expression through positive and negative feedback loops. During the night, PER and Timeless protein (TIM) levels rise in the cytoplasm, where the proteins heterodimerize, and then translocate into the nucleus to negatively regulate the transcription of their own genes and other genes. Negative regulation is effected by interactions with the transcription factors dClock (dCLK) and Cycle (CYC), which activate transcription of the per and tim genes and indirectly repress transcription of dClk in the absence of PER and TIM. Throughout the day, TIM levels do not accumulate in the nucleus because of TIM's degradation via a light-and cryptochrome-mediated degradation pathway (reviewed in reference 42).DBT, an ortholog of mammalian casein kinase Iε and casein kinase I␦ (CKIε/␦), regulates PER cytoplasmic and nuclear accumulation by triggering PER's degradation and regulating the timing of its nuclear accumulation (3,8,9,22,23,43,46,49). DBT's activity on PER is supplemented by the activity of CKII (2, 27, 28) and SGG (31) and is antagonized by a rhythmically expressed protein phosphatase (47). DBT may regulate other aspects of PER function (35) and ot...

In both mammals and fruit flies, casein kinase I has been shown to regulate the circadian phosphorylation of the period protein (PER). This phosphorylation regulates the timing of PER's nuclear accumulation and decline, and it is necessary for the generation of circadian rhythms. In Drosophila melanogaster, mutations affecting a casein kinase I (CKI) ortholog called doubletime (dbt) can produce short or long periods. The effects of both a short-period (dbt S ) and long-period (dbt L ) mutation on DBT expression and biochemistry were analyzed. Immunoblot analysis of DBT in fly heads showed that both the dbt S and dbt L mutants express DBT at constant levels throughout the day. Glutathione S-transferase pull-down assays and coimmunoprecipitation of DBT and PER showed that wild-type DBT, DBT S , and DBT L proteins can bind to PER equivalently and that these interactions are mediated by the evolutionarily conserved N-terminal part of DBT. However, both the dbt S and dbt L mutations reduced the CKI-7-sensitive kinase activity of an orthologous Xenopus laevis CKI␦ expressed in Escherichia coli. Moreover, expression of DBT in Drosophila S2 cells produced a CKI-7-sensitive kinase activity which was reduced by both the dbt S and dbt L mutations. Thus, lowered enzyme activity is associated with both short-period and long-period phenotypes.Many daily biochemical, physiological, and behavioral processes are termed circadian rhythms because they are temporally regulated by an endogenous circadian clock. While these endogenous clocks are usually synchronized by the environmental light-dark or temperature cycle, in the absence of environmental cues their oscillations persist with a period of approximately 24 h (reviewed in reference 48). A genetic analysis in Drosophila melanogaster, as well as in other model organisms, has revealed much about the molecular components and mechanism of the circadian clock (reviewed in reference 74). Recently, it has become clear that the mammalian circadian clock mechanism is quite similar to the Drosophila mechanism (reviewed in reference 4).Central to the molecular mechanism of Drosophila are the oscillations of the per, tim, and dClk gene products, which drive transcriptional-translational feedback loops (3,5,16,24,27,37,59,60,75; reviewed in reference 72). PER and TIM proteins accumulate during the night, become phosphorylated, dimerize, and enter the nucleus (14,18,22,29,47,50,56,61,70,76,77), where they negatively regulate transcription of their own mRNAs (11,16,17,26,27,43,63,71,75) and positively regulate transcription of the dClk mRNA (5, 24). Both the negative and positive feedback regulation are thought to result from direct protein-protein interactions of PER and/or TIM with a CLK/CYC transcription factor (11, 37, 71) which, in the absence of PER and TIM, activates transcription of per, tim, and vrille (3,7,11,16,17,24,26,37,43,54,71) and represses the transcription of dClk (15,23,24). Entrainment, or synchronization of the clock to light-dark cycles, is conferred in part by a crypt...

The Drosophila double-time (dbt) gene, which encodes a protein similar to vertebrate epsilon and delta isoforms of casein kinase I, is essential for circadian rhythmicity because it regulates the phosphorylation and stability of period (per) protein. Here, the circadian phenotype of a short-period dbt mutant allele (dbt(S)) was examined. The circadian period of the dbt(S) locomotor activity rhythm varied little when tested at constant temperatures ranging from 20 to 29 degrees C. However, per(L);dbt(S) flies exhibited a lack of temperature compensation like that of the long-period mutant (per(L)) flies. Light-pulse phase-response curves were obtained for wild-type, the short-period (per(S)), and dbt(S) genotypes. For the per(S) and dbt(S) genotypes, phase changes were larger than those for wild-type flies, the transition period from delays to advances was shorter, and the light-insensitive period was shorter. Immunohistochemical analysis of per protein levels demonstrated that per protein accumulates in photoreceptor nuclei later in dbt(S) than in wild-type and per(S) flies, and that it declines to lower levels in nuclei of dbt(S) flies than in nuclei of wild-type flies. Immunoblot analysis of per protein levels demonstrated that total per protein accumulation in dbt(S) heads is neither delayed nor reduced, whereas RNase protection analysis demonstrated that per mRNA accumulates later and declines sooner in dbt(S) heads than in wild-type heads. These results suggest that dbt can regulate the feedback of per protein on its mRNA by delaying the time at which it is translocated to nuclei and altering the level of nuclear PER during the declining phase of the cycle.