Introduction

The basic cellular oscillator which controls the various circadian rhythms of cells and organisms is often called the ‘circadian clock’. Knowledge about the mechanism of this ‘clock’ is emerging only slowly, progress being made particularly in the analysis of the period (per) gene and its products in Drosophila and other animal organisms (reviews: Rosbash & Hall, 1989; Young et al., 1989; Hall, 1990). Earlier approaches to the analysis of the clock mechanism were based mainly on phase shifting experiments with perturbing pulses or on biochemical evidence of oscillatory changes. These data indicated that protein synthesis, membrane potential, Ca2+-calmodulin and possibly other second messenger systems had roles in the clock mechanism or in the input pathways (reviews: Edmunds, 1988; Rensing and Hardeland, 1990). Evidence for the involvement of protein phosphorylation in this mechanism has also been indirectly derived from treatments with agents affecting kinase or phosphatase activities and from measurements of circadian rhythmic protein phosphorylation (Schroeder-Lorenz & Rensing, 1987; Techel et al., 1990). In addition, phase shifting pulses of light and elevated temperatures cause – among other effects – changes of protein phosphorylation within the cell (Lauter & Russo, 1990; Nover, 1990).

There are numerous ‘downstream’ processes, so called ‘hands’ of the clock, which also oscillate and cannot easily be distinguished from oscillating parts of the mechanism. Circadian rhythms in the phosphorylation state of phosphoenolpyruvate carboxylase (Carter et al., 1991; Nimmo, this volume), for example, may represent such a hand.