The temporal control of CONSTANS (CO) expression and activity is a key mechanism in photoperiodic flowering in Arabidopsis. FLAVIN-BINDING, KELCH REPEAT, F-BOX 1 (FKF1) protein regulates CO transcription, although the molecular mechanism is unknown. We demonstrate here that FKF1 controls the stability of a Dof transcription factor, CYCLING DOF FACTOR 1 (CDF1). FKF1 physically interacts with CDF1, and CDF1 protein is more stable in fkf1 mutants. Plants with elevated levels of CDF1 flower late and have reduced expression of CO. CDF1 and CO are expressed in the same tissues, and CDF1 binds to the CO promoter. Thus, FKF1 controls daily CO expression in part by degrading CDF1, a repressor of CO transcription.

The core mechanism of the circadian oscillators described to date rely on transcriptional negative feedback loops with a delay between the negative and the positive components . In plants, the first suggested regulatory loop involves the transcription factors CIRCADIAN CLOCK-ASSOCIATED 1 (CCA1) and LATE ELONGATED HYPOCOTYL (LHY) and the pseudo-response regulator TIMING OF CAB EXPRESSION 1 (TOC1/PRR1). TOC1 is a member of the Arabidopsis circadian-regulated PRR gene family . Analysis of single and double mutants in PRR7 and PRR9 indicates that these morning-expressed genes play a dual role in the circadian clock, being involved in the transmission of light signals to the clock and in the regulation of the central oscillator. Furthermore, CCA1 and LHY had a positive effect on PRR7 and PRR9 expression levels, indicating that they might form part of an additional regulatory feedback loop. We propose that the Arabidopsis circadian oscillator is composed of several interlocking positive and negative feedback loops, a feature of clock regulation that appears broadly conserved between plants, fungi, and animals.

RecQ helicase is important to homologous recombination and DNA repair in Escherichia coli. We demonstrate that RecQ helicase, in conjunction with RecA and SSB proteins, can initiate recombination events in vitro. In addition, RecQ protein is capable of unwinding a wide variety of DNA substrates, including joint molecules formed by RecA protein. These data are consistent with RecQ helicase assuming two roles in the cell; it can be (1) an initiator of homologous recombination, or (2) a disrupter of joint molecules formed by aberrant recombination. These findings also shed light on the function of the eukaryotic homologs of RecQ helicase, the Sgs1, Blm, and Wrn helicases.

Circadian clocks synchronize internal processes with environmental cycles to ensure optimal timing of biological events on daily and seasonal time scales. External light and temperature cues set the core molecular oscillator to local conditions. In Arabidopsis, EARLY FLOWERING 3 (ELF3) is thought to act as an evening-specific repressor of light signals to the clock, thus serving a zeitnehmer function. Circadian rhythms were examined in completely darkgrown, or etiolated, null elf3-1 seedlings, with the clock entrained by thermocycles, to evaluate whether the elf3 mutant phenotype was light-dependent. Circadian rhythms were absent from etiolated elf3-1 seedlings after exposure to temperature cycles, and this mutant failed to exhibit classic indicators of entrainment by temperature cues, consistent with global clock dysfunction or strong perturbation of temperature signaling in this background. Warm temperature pulses failed to elicit acute induction of temperatureresponsive genes in elf3-1. In fact, warm temperature-responsive genes remained in a constitutively "ON" state because of clock dysfunction and, therefore, were insensitive to temperature signals in the normal time of day-specific manner. These results show ELF3 is broadly required for circadian clock function regardless of light conditions, where ELF3 activity is needed by the core oscillator to allow progression from day to night during either light or temperature entrainment. Furthermore, robust circadian rhythms appear to be a prerequisite for etiolated seedlings to respond correctly to temperature signals.temperature signaling | temperature entrainment | luciferase | circadian rhythms | transcription T he rotation of Planet Earth creates predictable daily environmental fluctuations of light and dark along with concomitant oscillations in temperature. The circadian clock is an endogenous timekeeper that anticipates these predictable changes in the environment, confers rhythmic behavior to biological processes, and optimally phases biological activities to specific times of the day. Circadian clocks are widespread in nature, and processes under their control range from sleep-wake cycles in humans to daily expression of photosynthetic genes in plants. Clocks also time seasonal responses, such as the flowering transition in many plant species.Core molecular oscillators in eukaryotes incorporate interlocked transcription-translation feedback loops. In the model plant Arabidopsis thaliana, three such loops are critical to generation and maintenance of circadian rhythms. The loop first discovered is composed of the pseudoresponse regulator TOC1 (1) and two partially redundant Myb-like transcription factors, CCA1 (2) and LHY (3). Morning expression of CCA1 and LHY represses TOC1 expression by binding to its promoter (4), and circadian accumulation of TOC1 in the evening helps to induce CCA1 and LHY. A second morning-phased loop includes two TOC1-related proteins, PRR7 and PRR9 (5, 6). CCA1 and LHY induce PRR7 and PRR9 expression, whereas the two PRRs subseq...