Biochemical Modeling of an Autonomously Oscillatory Circadian Clock in Euglena

Abstract

Eukaryotic microorganisms, as well as higher animals and plants, display many autonomous physiological and biochemical rhythmicities having periods approximating 24 hours. In an attempt to determine the nature of the timing mechanisms that are responsible for these circadian periodicities, two primary operational assumptions were postulated. Both the perturbation of a putative element of a circadian clock within its normal oscillatory range and the direct activation as well as the inhibition of such an element should yield a phase shift of an overt rhythm generated by the underlying oscillator. Results of experiments conducted in the flagellate Euglena suggest that nicotinamide adenine dinucleotide (NAD+), the mitochondrial Ca2+-transport system, Ca2+, calmodulin, NAD+ kinase, and NADP+ phosphatase represent clock "gears" that, in ensemble, might constitute a self-sustained circadian oscillating loop in this and other organisms.

A Possible Second Role for Calmodulin in Biological Clock-Controlled Processes of Euglena

“…The ability of calmodulin antagonists to alter the phase of the photosynthesis rhythm was examined by exposing cells to 2 h pulses ofa particular antagonist. At the beginning of the transition from dark to light, when the culture was at a density of 8 x 104 cells ml1', the light intensity was adjusted to 3.5 1uE m-2 s-' for the following 4 (7). To ensure that a potential phase shift would be observed, each of the three antagonists were pulsed at six separate times during the h 7 to 26 of constant light conditions.…”

“…The biochemical basis for the biological clock is unknown and it is not certain whether cells have one master clock controlling all circadian rhythms, or whether multiple clocks control specific subsets ofrhythmic processes. The circadian rhythms ofEuglena have been extensively studied (3) and recently a feedback loop involving Ca2" and calmodulin was proposed in an attempt to define biochemically components of the clock controlling the Euglena cell division rhythm (7). The rationale for proposing calmodulin involvement was based on the observation that exposure to a calmodulin antagonist results in phase shifting (timing advances or delays) of the monitored cell division rhythm (7).…”

“…The circadian rhythms ofEuglena have been extensively studied (3) and recently a feedback loop involving Ca2" and calmodulin was proposed in an attempt to define biochemically components of the clock controlling the Euglena cell division rhythm (7). The rationale for proposing calmodulin involvement was based on the observation that exposure to a calmodulin antagonist results in phase shifting (timing advances or delays) of the monitored cell division rhythm (7). If the proposed feedback loop represents a master clock in Euglena, then all rhythms using the master clock should be phase shifted by the application of calmodulin inhibitors.…”

A mutant of Neurospora crassa that has a long lag phase in low-calcium medium

“…We concluded that the metabolism of calcium is important for clock functions from the late subjective day phase to the early subjective night phase (Nakashima, 1984(Nakashima, , 1986. Important roles for calcium in clock functions have also been indicated in other organisms, such as Aplysia (Eskin & Corrent, 1977) and Euglena (Goto et al, 1985;Tamponnet & Edmunds, 1991).…”

Ritmos circadianos em Gracilaria birdiae (Rhodophyta): oscilação do desempenho fotossintético e caracterização enzimática da nitrato redutase

“…Gonyaulax (hoje Linguludinium), Euglena, Chlamydomonas e Acetabularia (DRIESSCHE; BONOTTO, 1969;GOTO et al, 1985;LOPES et al, 2002;MORSE et al, 1989). Desses estudos foi concluído que osciladores pós-traducionais também existem em eucariotos.…”