~ ARTICLE ~ The circadian clock in the prokaryote

The term ‘circadian rhythm ’ is used to denote an endogenous oscillation in a behavioural or biochemical phenomenon which (1) persists for at least several days under uniform environmental conditions, (2) has a periodicity of about 24 h (circa = approximately; diem = day) and (3) is unique amongst rhythms in living organisms in that it has a period of oscillation that shows a high degree of temperature compensation, so that it changes but little at different constant ambient tempera-tures (Wilkins, 1992). The term ‘diurnal rhythm ’ is reserved for conditions in which there is a direct input of external periodic signals. Consequently ‘ diurnal rhythms ’ do not manifest themselves under uniform environmental conditions. Many biological parameters are coupled to the circadian clock mechanism and therefore exhibit daily rhythmic fluctuations (Edmunds, 1988). Some parameters which have been intensively studied include conidiatio