CIRCADIAN RHYTHMS - Cyanobacteria - page 321

Figure 5: The core PTO is embedded in a larger TTFL. The PTO is linked to the damped TTFL (indicated by the pink background
circle) by transcription and translation of the kaiABC cluster. Global gene expression is mediated by rhythmic modulation
of the activity of all promoters, including those driving the expression of the central clock gene cluster, kaiABC ( = ABC in
fi gure). Rhythmic DNA torsion and/or transcriptional factor activity (e.g. RpaA/SasA) modulate global promoter activities.
Cyclic changes in the phosphorylation status of KaiC that mediate the formation of the KaiBNKaiC complex regulate DNA
topology/transcriptional factors. The PTO (cycle connected by lavender arrows in upper right quadrant) is determined by
KaiC phosphorylation as regulated by interactions with KaiA and KaiB. Robustness is maintained by synchronization of
KaiC hexameric status via monomer exchange (Ito et al ., 2007; Mori et al ., 2007). Monomer exchange is depicted in the fi gure
by ''dumbbell'' KaiC monomers exchanging with KaiC hexamers in the middle of the PTO cycle; phase-dependent rate of
monomer exchange is indicated by the thickness of the double-headed black arrows. The shade of KaiC hexamers (dark versus
light blue) denotes conformational changes that roughly equate to kinase versus phosphatase forms. New synthesis of KaiC
feeds into the KaiABC oscillator as non-phosphorylated hexamers or as monomers that exchange into pre-existing hexamers.
If the new synthesis of KaiC occurs at a phase when hexamers are predominantly hypo-phosphorylated, the oscillation of
KaiC phosphorylation is reinforced (enhanced amplitude). If on the other hand, new synthesis of unphosphorylated KaiC
happens at a phase when hexamers are predominantly hyper-phosphorylated, this leads to an overall decrease in the KaiC
phosphorylation status, thereby altering the phase of the KaiABC oscillator (phase shift) and/or reducing its amplitude. With
the kind permission of C. H. Johnson, Department of Biological Sciences, Vanderbilt University, Nashville, Tennessee, USA
[Qin et al . (2010) PLoS Biol 8(6) : e1000394. doi:10.1371/journal.pbio.1000394] doi:10.1371/journal.pbio.1000394.g007.
Color image of this figure appears in the color plate section at the end of the topic.
redox state of the plastoquinone pool inside the cells. The ligand-binding role of the pseudoreceiver
domain of CikA has been demonstrated by its binding to quinone directly. As already mentioned,
CikA associates with Kai proteins and infl uences the phosphorylation state of KaiC during the
resetting of circadian phase by a dark pulse. Thus it seems that the resetting of clock in S . elongatus
PCC 7942 is metabolism-dependent and the oscillator KaiC responds to the signals transmitted by
CikA. The three domains, i.e. GAF, HPK and PsR are shown to be necessary for the functioning
of CikA whereas N-terminal portion seems to be not necessary in vivo . The localization of CikA