Persisting circadian rhythm of cell division in a photosynthetic mutant of Euglena

Circadian clocks and cell division: what's the pacemaker?

Evolution has selected a system of two intertwined cell cycles: the cell division cycle (CDC) and the daily (circadian) biological clock. The circadian clock keeps track of solar time and programs biological processes to occur at environmentally appropriate times. One of these processes is the CDC, which is often gated by the circadian clock. The intermeshing of these two cell cycles is probably responsible for the observation that disruption of the circadian system enhances susceptibility to some kinds of cancer. The core mechanism underlying the circadian clockwork has been thought to be a transcription & translation feedback loop (TTFL), but recent evidence from studies with cyanobacteria, synthetic oscillators and immortalized cell lines suggests that the core circadian pacemaking mechanism that gates cell division in mammalian cells could be a post-translational oscillator (PTO).

Clocks in the green lineage: comparative functional analysis of the circadian architecture of the picoeukaryote ostreococcus

Biological rhythms that allow organisms to adapt to the solar cycle are generated by endogenous circadian clocks. In higher plants, many clock components have been identified and cellular rhythmicity is thought to be driven by a complex transcriptional feedback circuitry. In the small genome of the green unicellular alga Ostreococcus tauri, two of the master clock genes Timing of Cab expression1 (TOC1) and Circadian Clock-Associated1 (CCA1) appear to be conserved, but others like Gigantea or Early-Flowering4 are lacking. Stably transformed luciferase reporter lines and tools for gene functional analysis were therefore developed to characterize clock gene function in this simple eukaryotic system. This approach revealed several features that are comparable to those in higher plants, including the circadian regulation of TOC1, CCA1, and the output gene Chlorophyll a/b Binding under constant light, the relative phases of TOC1/CCA1 expression under light/dark cycles, arrhythmic overexpression phenotypes under constant light, the binding of CCA1 to a conserved evening element in the TOC1 promoter, as well as the requirement of the evening element for circadian regulation of TOC1 promoter activity. Functional analysis supports TOC1 playing a central role in the clock, but repression of CCA1 had no effect on clock function in constant light, arguing against a simple TOC1 /CCA1 one-loop clock in Ostreococcus. The emergence of functional genomics in a simple green cell with a small genome may facilitate increased understanding of how complex cellular processes such as the circadian clock have evolved in plants.

Circadian control of cell division in unicellular organisms

Cell division cycles and circadian rhythms are major periodic phenomena in organisms. Circadian oscillators control biochemical, physiological, and behavioral events in a wide range of living systems including almost all eukaryotes that have been tested and some prokaryotes-in particular, the cyanobacteria. Gating of cell division is one of the key processes that has been reported to be regulated by circadian clocks in many organisms. We survey studies of the circadian control of cell division in eukaryotic microorganisms and introduce recent progress on understanding the interaction between circadian rhythms and cell division cycles in cyanobacteria.

Effects of light and acetate on the liberation of zoospores by a mutant strain ofChlamydomonas reinhardtii

In light-dark-synchronized cultures of the unicellular green algaChlamydomonas reinhardtii, release of zoospores from the wall of the mother cell normally takes place during the second half of the dark period. The recently isolated mutant 'ls', however, needs light for the liberation of zoospores when grown photoautotrophically under a 12 h light-12 h dark regime. The light-induced release of zoospores was found to be prevented by addition of the photosystem-II inhibitor 3-(3',4'-dichlorophenyl)-1,1-dimethylurea. Furthermore, light dependence of this process was shown to be abolished when the mutant 'ls' was grown either photoautotrophically under a 14 h light-10 h dark regime or in the presence of acetate. Our findings indicate that the light-dependency of zoospore liberation observed in cultures of this particular mutant during photoautotrophic growth under a 12 h light-12 h dark regime might be attributed to an altered energy metabolism. The light-induced release of zoospores was found to be prevented by addition of cycloheximide or chloramphenicol, antibiotics which inhibit protein biosynthesis by cytoplasmic and organellar ribosomes, respectively. Actinomycin D, an inhibitor of RNA synthesis, however, did not affect the light-induced liberation of zoospores.Sporangia accumulate in stationary cultures of the mutant 'ls'. Release of zoospores was observed when these sporangia were collected by centrifugation and incubated in the light after resuspension in fresh culture medium. Since liberation of zoospores was not observed after dilution of the stationary cultures with fresh culture medium, we suppose that components which interfere with the action of the sporangial autolysin are accumulated in the culture medium of the mutant 'ls'.

The Chlamydomonas cell cycle is regulated by a light/dark-responsive cell-cycle switch

Cultures of the unicellular green alga Chlamydomonas reinhardii can be synchronized by light/dark cycling not only under photoautotrophic but also under mixotrophic growth conditions. We observed that cultures synchronized in the presence of acetate continue to divide synchronously for one cell-cycle period when transferred to heterotrophic growth conditions. This finding enabled us to investigate the differential effects of light on cell growth and cell division. When cells were exposed to continuous light at the beginning of the growth period they entered the division phase earlier than dark-grown cells as a consequence of an increased growth rate. Illumination at the end of the growth period, however, caused a considerable delay in cell division and an extended growth period. The light-induced delay in cell division was also observed in the presence of 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU), an inhibitor of photosystem II. This finding demonstrates that cell division is directly influenced by a light/dard-responsive cell-cycle switch rather than by light/dark-dependent changes in energy metabolism. The importance of this light/dark control to the regulation of the Chlamydomonas cell cycle was investigated in comparison with other control mechanisms (size control, time control). We found that the light/dard-responsive cell-cycle switch regulates the transition from G1-to S-phase. This control mechanism is effective in cells which have attained the commitment to at least one round of DNA replication and division but have not attained the maximal cell mass which initiates cell division in the light.

Chronobiology at the cellular and molecular levels: models and mechanisms for circadian timekeeping

This review considers cellular chronobiology and examines, at least in a superficial way, several classes of models and mechanisms that have been proposed for circadian rhythmicity and some of the experimental approaches that have appeared to be most productive. After a brief discussion of temporal organization and the metabolic, epigenetic, and circadian time domains, the general properties of circadian rhythms are enumerated. A survey of independent oscillations in isolated organs, tissues, and cells is followed by a review of selected circadian rhythms in eukaryotic microorganisms, with particular emphasis placed on the rhythm of cell division in the algal flagellate Euglena as a model system illustrating temporal differentiation. In the ensuing section, experimental approaches to circadian clock mechanisms are considered. The dissection of the clock by the use of chemical inhibitors is illustrated for the rhythm of bioluminescence in the marine dinoflagellate Gonyaulax and for the rhythm of photosynthetic capacity in the unicellular green alga Acetabularia. Alternatively, genetic analysis of circadian oscillators is considered in the green alga Chlamydomonas and in the bread mold Neurospora, both of which have yielded clock mutants and mutants having biochemical lesions that exhibit altered clock properties. On the basis of the evidence generated by these experimental approaches, several classes of biochemical and molecular models for circadian clocks have been proposed. These include strictly molecular models, feedback loop (network) models, transcriptional (tape-reading) models, and membrane models; some of their key elements and predictions are discussed. Finally, a number of general unsolved problems at the cellular level are briefly mentioned: cell cycle interfaces, the evolution of circadian rhythmicity, the possibility of multiple cellular oscillators, chronopharmacology and chronotherapy, and cell-cycle clocks in development and aging.

Clocked cell cycle clocks

The cell division cycle of both mammalian cells and microorganisms, which apparently has both deterministic and probabilistic features, is a clock of sorts in that the sequence of events that comprise it measures time under a given set of environmental conditions. The cell division cycle may itself be regulated by a programmable clock that, under certain conditions, can generate circadian periodicities by interaction with a circadian pacemaker. These clocks must insert time segments into the cell division cycle in order to generate the observed variability in cellular generation times.

Regulation of the Chlamydomonas cell cycle by light and dark

By growing cells in alternating periods of light and darkness, we have found that the synchronization of phototrophically grown Chlamydomonas populations is regulated at two specific points in the cell cycle: the primary arrest (A) point, located in early G1, and the transition (T) point, located in mid-G1. At the A point, cell cycle progression becomes light dependent. At the T point, completion of the cycle becomes independent of light. Cells transferred from light to dark at cell cycle position between the two regulatory points enter a reversible resting state in which they remain viable and metabolically active, but do not progress through their cycles. The photosystem II inhibitor dichlorophenyldimethylurea (DCMU) mimics the A point block induced by darkness. This finding indicates that the A point block is mediated by a signal that operates through photosynthetic electron transport. Cells short of the T point will arrest in darkness although they contain considerable carbohydrate reserves. After the T point, a sharp increase occurs in starch degradation and in the endogenous respiration rate, indicating that some internal block to the availability of stored energy reserves has now been released, permitting cell cycle progression.

The coupling effects of some thiol and other sulfur-containing compounds on the circadian rhythm of cell division in photosynthetic mutants of Euglena

Previous work has demonstrated a persisting, free-running, circadian rhythm of cell division in the P4ZUL photosynthetic mutant of the alga Euglena gracilis Klebs (Strain Z) Pringsheim grown organotrophically in continuous light or darkness at 19 degrees C following prior synchronization by a repetitive LD:10,14 light cycle. A similar circadian rhythmicity has been recently discovered in the W6ZHL heat-bleached and the Y9ZNalL naladixic acid-induced mutants of Euglena grown under comparable conditions. Over extended timespans, however, these mutants appear to gradually lose first their ability to display persisting overt rhythms, and then even their capability of being entrained by imposed LD cycles. These properties can be restored by the addition of certain sulfur-containing compounds to the medium including cysteine, methionine, dithiothreital, sodium monosulfide, sodium sulfite, and sodium thiosulfate, as well as thioglycolic [mercaptoacetic] acid. The implications of these findings toward biological clock mechanisms are discussed: It appears that some sort of coupling process is operating as opposed to the initiation of an underlying oscillation.

[Comparison of specific synthesis of RNA in synchronized Euglena gracilis (Z) grown on lactate medium in the dark and in the light, and a study of the mitochondria]

Synchronization of Euglena gracilis (Z) on lactate medium is shown to be independent of illumination. The existence of a mitochondrial cycle in lightgrown as well as in dark-grown Euglena is demonstrated. When RNA synthesis is studied by pulse labeling with tritiated uracil in synchronously growing cells, a discontinuous RNA synthesis is found. Two peaks of preferential RNA synthesis in dark-grown cells and three peaks in light-grown cells are seen; the significance of the third peak of RNA synthesis in light-grown Euglena is discussed.

Photoinduced division synchrony in permanently bleached Euglena gracilis

Ultraviolet light-induced, bleached Euglena clones exhibit synchronous steps of cell division in response to daily cycles of light and dark. The cyclic division activity, in the bleached cells, will persist in constant lighting conditions with a period, independent of temperature, of about 24 h. This persisting rhythm of cell division supports the hypothesis that this phase of the cell cycle may be coupled to the fluctuations of the endogenous circadian clock of the cell.Newly isolated bleached clones are sensitive to light in their growth rates and metabolic characteristics, showing light induced difference in substrate-stimulated respiration, and production of the polyglucan, paramylon. After repeated subculturing of a bleached clone the photosensitivity of the metabolic characteristics and of the growth rate are diminished along with the ability to photo-entrain division synchrony. Division control and the induction of cell synchrony in this organism apparently involve both the temporal influence of the endogenous cell clock and one or more other photosensitive reactions in the metabolism of the cell.A unique culture mixing technique utilizing the bleached Euglena, failed to support the hypothesis of the involvement of intercellular communication in the maintenance of cell synchrony in constant lighting conditions.

Phasing of cell division by temperature cycles in Euglena cultured autotrophically under continuous illumination

Autotrophic cultures of Euglena in continuous light (LL) were exposed to temperature cycles spanning different temperature intervals in the physiological range. Each cycle consisted of a regular alternation between equal phases of two temperatures differing by 7°. Different temperature combinations varied in the degree to which they were capable of phasing cell division. The most effective combinations tested, 18/25° and 28/35°, produced almost as good a synchrony as has been observed from the use of light/dark cycles (at constant temperature) in this system. Both batch and chemostat cultures with a wide range of generation times were phased, and cycles with period lengths ranging from 8 to 24 hr appeared to be equally effective.Dry weight of cells per milliliter of sample was found to increase uniformly in phased cultures, indicating that most biosynthetic processes continue in such cultures even while all division is inhibited. Phasing cannot, therefore, be explained as a simple growth inhibition by the less favorable temperature of the cycle.The average generation times of cultures phased by 12,12 hr cycles were shorter than the "expected" times calculated from the results of the corresponding pairs of constant temperature experiments, indicating that temperature cycles have an overall accelerating effect on the growth of Euglena in continuous light. This and other evidence suggests that temperature cycles may be acting as Zeitgeber for Euglena in LL even though many trials revealed no persistence of a cell division rhythm in conditions of constant temperature and LL following temperature-cycle entrainment.