The cyanobacterium Synechococcus elongatus builds a circadian clock on an
oscillator comprised of three proteins, KaiA, KaiB, and KaiC, which can
recapitulate a circadian rhythm of KaiC phosphorylation in vitro. The molecular
structures of all three proteins are known, and the phosphorylation steps of
KaiC, the interaction dynamics among the three Kai proteins, and a weak
ATPase activity of KaiC have all been characterized. A mutant of a clock gene in
the input pathway, cikA, has a cell division defect, and the circadian clock
inhibits the cell cycle for a short period of time during each cycle. However, the
interaction between the circadian cycle and the cell cycle and the molecular
mechanisms underlying it have been poorly understood. In addition, the
subcellular localization of clock proteins and possible localization dynamics,
which are critical in the timing circuit of eukaryotic clock systems and might also
shed light on the interaction between circadian cycle and cell cycle, have remained largely unknown. A combination of genetics, cell biology, and
microscopy techniques has been employed to investigate both questions.
This work showed that the cell division defect of a cikA mutant is a function of
the circadian clock. High ATPase activity of KaiC coincides with the inhibition of
cytokinesis by the circadian clock. CikA likely represses KaiC's ATPase activity
through an unknown protein, which in cikA's absence stimulates both the
ATPase and autokinase activities independently of KaiA or KaiB. SasA-RpaA
acts as an output in the control of cell division, and the localization of FtsZ is the
target, although it still remains to be seen how RpaA, directly or indirectly,
inhibits FtsZ localization.
The project also showed that clock proteins are localized to the cell poles.
KaiC is targeted to the cell pole in a phosphorylation-dependent manner. KaiB
and CikA are also found at the poles independently of KaiC. KaiA likely only
localizes to the cell pole during the dephosphorylation phase, which is
dependent on both KaiB and KaiC, specifically on the phosphorylation of KaiC at
S431.
Overall, significant progress was made in both areas and this project sheds
light on how the circadian oscillator operates in cyanobacterial cells and
interacts with another fundamental cellular function.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/ETD-TAMU-2008-12-197 |
| Date | 2008 December 1900 |
| Creators | Dong, Guogang |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | thesis, text |
| Format | application/pdf |
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