CIRCADIAN RHYTHMS - Cyanobacteria - page 320

fact some of the mathematical models also supported this idea (Kurosawa et al ., 2006b). However,
convincing evidences for the operation of two KaiC-based oscillators, i.e. the fi rst one by the PPC and
the second one by TTFL have been put forward by Kitayama et al . (2008). The KaiA overexpression
mutants ( ox-kaiA ) of S . elongatus PCC 7942 showed normal temperature-compensated circadian
oscillation, despite KaiC being present in a phosphorylated but in an arrested state. It means even
when the PPC is not allowed to operate, there is an additional input of KaiC proteins through TTFL.
Kitayama et al . (2008) used three types of KaiC mutants to demonstrate the existence of TTFL in
the absence of PPC. The fi rst category of KaiC mutants are those in which Ser431 and Thr432 have
been substituted by Ala (isolated by Nishiwaki et al ., 2004) and in this respect KaiC in these mutants
constitutively exists always in an hypophosphorylated state but kaiBC expression in these mutants
fl uctuated arrhythmically. The second category of KaiC mutants are those in which Ser431 and Thr432
have been substituted by Glu. Since Glu mimics phosphorylation at these sites and so KaiC is said
to exist in a hyperphosphorylated state. These mutants showed a dampened but a clear rhythm. The
levels of KaiB and KaiC proteins also fl uctuated rhythmically. An autokinase defi cient mutant with
a change in Lys294 to His294 (isolated by Hayashi et al ., 2004) produced unphosphorylated KaiC.
All these point to the functioning of a second KaiC oscillator dependent on TTFL that is also equally
important for the maintenance of circadian rhythms in cyanobacteria. Qin et al . (2010) preferred
to call the PPC-based KaiC oscillator as the post-translational oscillator (PTO) and considered it
as the core pacemaker and the TTFL as the slave oscillator. Based on the mathematical modelling
and biochemical evidences they concluded that: (i) the core oscillator can function independently
of TTFL; (ii) any inhibition of PTO/PPC would adversely affect TTFL; (iii) the slave oscillator can
make additional inputs of KaiC protein into the core oscillator; and (iv) the core oscillator is coupled
to the slave oscillator. The importance of TTFL has been emphasized when cell doubling times are
faster and fall below 24 h. In such cells the PTO is unable to meet the demand to maintain circadian
rhythms but a feedback of KaiC proteins from TTFL helps to cope up the situation (Fig. 5). Zwicker
et al . (2010) used stochastic simulations to show that PTO and TTFL are intertwined and work in
tandem to maintain robust circadian rhythms in cyanobacteria experiencing high growth conditions.
These developments have been summarized (Johnson et al ., 2008, 2011).
IV. INPUT PATHWAYS
Phase resetting studies on mutant AMC149 led to the alternation of light and dark phases (Kondo et
al ., 1993). Apart from light, other environmental signals like temperature, pH, osmotic environment
may also play a signifi cant role in phase setting input to the circadian clock in S . elongatus PCC 7942.
However, the participation of photoreceptors (phytochome or other photoreceptor) cannot be ruled
out. In this connection, the observations of Schmitz et al. (2000) are noteworthy who identifi ed cikA
(circadian input kinase) gene sequence in S . elongatus PCC 7942 that encodes a bacteriophytochome
related kinase. This has been identifi ed as a key component of the input pathway that supplies
environmental information to the circadian oscillator in S . elongatus PCC 7942. It possesses three
domains a GAF, a histidine protein kinase (HPK) and a pseudo-receiver domain (PsR) similar to
those of response regulator family (Schmitz et al ., 2000). Further it was observed that inactivation of
CikA abolishes the ability of the clock to be reset by a dark pulse. The possibility of CikA binding to
the chromophore has been ruled out due to the absence of the conserved amino acid residues in the
chromophore binding domain and in vivo binding experiments were negative (Matsuda et al ., 2003).
The manner in which CikA functions and the type of environmental signal it perceives has come
to light by the studies of Ivleva et al . (2006) who presented evidences that it senses not light but the