CIRCADIAN RHYTHMS - Cyanobacteria - page 324

Liu et al . (1995) proposed a model for explaining non-specifi c circadian control and a circadian
regulation by specifi c trans factors. It has been suggested that Class 1 specifi c Cis -elements are
turned on during the day by a Class 1-specifi c trans factor. Likewise, Class 2 specifi c Cis elements
would be turned on at night by a Class 2-specifi c trans factor. Though it is diffi cult to imagine the
existence of trans factors for all the large number of genes under the circadian control, it is suggested
that there might be involvement of some global factors. Moreover, the discovery of a gene whose
altered expression signifi cantly lowered the amplitude of the luminescence rhythm driven by some
promoters such as psbAI but not of luminescence rhythm driven by other promoters such as that
of purF . These observations point towards the existence of subsets of clock-controlled genes in S .
elongatus PCC 7942.
Katayama et al. (1999) identifi ed a gene involved in the output pathway of S . elongatus PCC
7942 while examining a transposon Tn5 - generated mutant tnp6 that is affected in both amplitude
and phasing of the psbAI:luxAB circadian expression rhythm. They introduced a derivative of
TN5 into the chromosomes of reporter strains in which cyanobacterial promoters drive the Vibrio
harveyi luxAB genes. As a result of which the oscillation of bioluminescence could be measured as
a function of circadian gene expression. This mutant gene has been designated as cpmA (circadian
phase modifi er) and is shown to change the circadian phasing of promoter activity for one of the
genes that encodes a central clock component ( kaiA :: luxAB ) but it had little effect on the other two
clock genes ( kaiB::luxAB ). They further concluded that the coordinated expression of Kai genes is
not essential for the circadian time keeping in Synechococcus .
i) Role of light and dark : Entrainment is generally defi ned as the matching of the period of biological
clock exactly equal to the environmental cycle. The primary signals of entrainment used are light
and dark cycles that can set the phase of the circadian clock. Though phytochromes, rhodopsins or
cryptochromes are the photopigments involved in circadian entrainment in other organisms, in case
of cyanobacteria the photopigments involved in the process are poorly understood (Johnson, 1995;
Roenneberg and Foster, 1997; Johnson and Golden, 1999). Blue and red lights are most effective in
setting the phase of the cyanobacterial clock. However, it was not possible to reverse the phase of
the clock in red or far-red lights, respectively. The action spectrum does not coincide with either that
of photosynthesis or phytochrome. The available evidences suggest that there are certain specifi c
unknown pigments that perceive the signals in the input pathway of the circadian clock of S . elongatus
PCC 7942 and Synechococcus RF-1.
In cyanobacteria, the rhythmicity of the circadian cycle has been tested invariably in LL condition.
The next question that emerges is whether light is required to run the circadian clock? Kondo et
al . (1994) used light pulses that can reset the phase of the clock in DD in the photoautotrophic
S . elongatus PCC 7942. They observed that circadian clock continued to function even in DD. Some
other studies examined whether light is necessary for the continued functioning of the circadian
clock or only certain metabolic rate has to be maintained for the clock to express. The presence of
circadian rhythms in the cyanobacterium Synechocystis sp. strain PCC 6803 was shown by Aoki et
al. (1995) by the rhythmic expression of dnaK gene (DnaK is a member of well-conserved heat shock
proteins that plays a protective role in supporting growth at high temperatures beyond the normal
physiological range). They fused a promoterless luxAB gene set downstream the promoter segment
of the Synechocystis dnaK gene and introduced it into a specifi c site of the Synechocystis chromosome.
Aoki et al . (1997) further showed that the circadian rhythms persisted even in DD. They selected
Synechocystis sp. strain PCC 6803 and Cyanothece sp. ATCC 51142 that can grow heterotrophically
on glucose and glycerol, respectively for such studies. Synechocystis can grow heterotrophically on