CIRCADIAN RHYTHMS - Cyanobacteria - page 313

Figure 2: Molecular feedback loops of cyanobacteria. A cluster of KaiABC genes controls circadian rhythms in cyanobacteria.
KaiA gene product acts as a positive regulator for KaiBC transcription, while KaiBC products along with other proteins inhibit
their own transcription. With kind permission of V. K. Sharma, Chronobiology Laboratory, Evolutionary and Organismal
Biology Unit, Jawaharlal Nehru Centre for Advanced Scientifi c Research, Jakkur, PO Box 6436, Bangalore 560 064, Karnataka,
India [Paranjpe, D. A. and Sharma, V. K. (2005) J Circadian Rhythms 3 : 7 doi:10.1186/1740-3391-3-7].
PCC 7942 with EMS. This mutant designated as pr1 (for KaiC phase response 1) exhibited normal
growth as well as circadian rhythm similar to the wild-type. Interestingly, however, the rhythmicity
in KaiC levels and its phosphorylation rates were greatly affected due to the mutation. Additionally,
KaiC phosphorylation state was reduced along with decreased autoregulatory activity of KaiC.
These studies emphasize the importance of protein phosphorylation cycle (PPC) of KaiC in resetting
the cyanobacterial clock. In support of their above studies, Kurosawa et al . (2006b) presented a
mathematical model for gene protein dynamics that can generate a sustained oscillation in both DD
and LL conditions. According to them, KaiC phosphorylation seems to be the basis for the sustained
oscillations in the absence of transcriptional or translational regulation.
Negative feed back of KaiC on its own promoter is another similarity that the cyanobacterial
circadian clock shares with the eukaryotic clock proteins. Continuous over expression of KaiC
represses kaiBC promoter (P kaiBC ) and represses the rhythms of all promoters in the S . elongatus
PCC 7942 genome. Two classes of kaiC repression have been identifi ed by Nakahira et al . (2004). The
fi rst class termed as “high amplitude” expression, exhibited by 5-10% promoters including P kaiBC ,
is clock-dominated. The second type of response, exhibited by 90-95% of the promoters, has been
termed as “low amplitude”. This is a clock-modulated response where the genes have signifi cant
basal activity and are rhythmically modulated by KaiABC oscillator. KaiC thus coordinates the
genome-wide gene expression as suggested by Johnson (2004). Genes that encode the proteins
intrinsically associated with the circadian clock are associated with the second class. When KaiC
was kept under the control of a synthetic promoter P trc from E . coli , KaiB and KaiC could restore the
complete circadian rhythm in KaiBC inactivated arrhythmic strain of S. elongatus PCC 7942.
In contrast to autoregulatory nature of the circadian feed back loops in eukaryotes, in
cyanobacteria the Kai promoters can be functionally replaceable. The replacement of P kaiBC has
been reported fi rst by Xu et al. (2003) followed by the replacement of P kaiA and P kaiBC by Nakahira