CIRCADIAN RHYTHMS - Cyanobacteria - page 304

input pathway, central oscillator and output pathway. The input pathway resets the phase of the
oscillation by responding to external/environmental signals. Of these, the light/dark cycle is the
most predominant external cycle and a major environmental stimulus for the synchronization of
circadian clock. The central oscillator (also designated as central pacemaker) receives signals through
the input pathway and generates the oscillation. The output pathway translates the oscillation into
behavioural and physiological rhythms. The circadian rhythms are exhibited by almost all groups of
organisms from prokaryotes to eukaryotes. These daily rhythms regulate a wide variety of biological
activities that help the organisms to adapt to daily changes in the environmental conditions.
Three important criteria are characteristic of circadian rhythms. Firstly, these persist in constant
environmental conditions of temperature, light or darkness with a period of 24 h. That is they have
an inherent periodicity of 24 h. Secondly, these rhythms are temperature compensated suggesting
that these remain constant over a permissive range of temperatures. Thirdly, these endogenous
rhythms can be tuned to oscillate with exactly 24 h period. This is also known as entrainment that
can be achieved by the variation of light/dark and/or temperature cycles. This enables the living
organisms to keep track of time in their local environment to exactly 24 h daily cycles.
Circadian rhythms were for the fi rst time detected in 1729 by a French astronomer, Jean Jacques
d'Ortous de Marian who observed that daily leaf movements of heliotrope persisted even when
the plant was kept in dark for several days. That is the leaves of the plant continued to open during
the day and close at night despite the absence of sunlight. But it was Erwin Bünning's work, on
leaf movements in Phaseolus that formed the foundation for circadian (biological) clock research
(Bünning, 1973). Recognizing the importance of circadian clock in the biological world, many
experimental systems have been developed for elucidating the biochemical, molecular and genetic
basis of the circadian rhythms. Although mutational studies on eukaryotic groups of organisms such
as algae ( Chlamydomonas reinhardtii ), fungi ( Neurospora crassa ), insects ( Drosophila melanogaster ), fi shes
(hamster), mammals (mouse) and plants ( Arabidopsis thaliana ) have been made, only D . melanogaster
and N . crassa have been amenable for a mutational analysis of the genes governing circadian rhythms
(Golden et al ., 1997). Thus in general, these studies led every one to believe that the circadian rhythms
are characteristic of eukaryotes alone. Moreover, it was generally thought that the prokaryotes are
entirely devoid of these rhythms. The idea that the prokaryotes either unicellular or multicellular
do not show circadian rhythms has gained strength to such an extent that many workers held it as
a dogma and excluded prokaryotes while proposing models for circadian mechanisms (Johnson et
al ., 1996).
The variations in response to light and dark cycles can be plotted into phase response curves. The
magnitude of phase shift as a result of the application of a given stimulus is plotted at different points
of time during the entire period of circadian cycle. The phase response curves show peaks alternating
with troughs coinciding with the alternating light:dark (L:D) cycles during the entrainment period.
These persist even after the cells are withdrawn and are subjected to a continuous light (LL) or
continuous dark (DD) period. This period through which the rhythms continue to be expressed is
known as free-running period. These are helpful in understanding the circadian oscillator.
Despite many odds, the persistent quest for identifying circadian rhythms in prokaryotic
systems continued. It is in this context that the cyanobacteria emerged as suitable experimental
systems for unraveling the molecular mechanism of circadian rhythms. However, the complete
genome sequencing of Synechocystis sp. strain PCC 6803 (Kaneko et al ., 1996) paved the way for the
identifi cation of Synechococcus sp. strain PCC 7942 (with a genome size of 2.7 mb almost equivalent
to the genome size of Escherichia coli ) as the novel experimental system for the study of circadian
rhythms.