Circadian changes in cyclic AMP levels in synchronously dividing and stationary-phase cultures of the achlorophyllous ZC mutant of Euglena gracilis

Abstract

Oscillations in adenosine 3′,5′-cyclic monophosphate (cyclic AMP) level have been proposed to be part of the biochemical feed-back loop(s), or ‘clock(8)’, believed to underlie circadian rhythmicity. This possibility has been examined for a cellular circadian oscillator in synchronously dividing (or nondividing) cultures of the photosynthesis- deficient ZC mutant of the alga Euglena gracilis Klebs (Z). We have demonstrated a bimodal, autonomously oscillating, circadian variation of cyclic AMP content in this unicell. Rhythmic changes of the cyclic AMP level, which may reflect the transition of the cell population through the different phases of the cell division cycle (CDC) in division-phased cultures, also persisted after the culture medium had become limiting and the cells had stopped dividing. We have also shown that the free-running, circadian oscillation of cyclic AMP content displayed by nondividing cells in continuous darkness could be phase-shifted by a light signal (a property inherent to most circadian systems), in a manner that could be predicted from the phase-response curve previously obtained for the cell division rhythm in the ZC mutant. These results suggest a possible role for cyclic AMP, either as an element of the coupling pathway for the control of the CDC by the circadian oscillator, or as a ‘gear’ of the clock itself.