CLOCK GENES IN CYANOBACTERIA

It was originally thought that prokaryotes do not have circadian rhythms. However, the existence of prokaryotic circadian rhythms has been proven (Golden, 1997). The most well studied circadian clock is in Cyanobacteria. 

Cyanobacteria contain 3 clock genes, called kaiA, kaiB and kaiC. All 3 of these genes are found adjacent to each other, and the expression of all 3 is rhythmic (Harmer et al. 2001). The levels of mRNA from all of these kai genes peaks towards the end of the day. kaiB and kaiC share a single promoter and are transcribed together. Levels of KaiB and KaiC protein peak approximately 5 hours later, indicating some post-transcriptional control. Levels of KaiA do not cycle. 

KaiA, KaiB and KaiC interact with each other. Overexpression or loss of these genes has effects on the others:

kaiA: Overexpression – Enhances expression of kaiBC

Null- Decrease in kaiBC transcripts.

kai C: Overexpression – Suppresses expression of kaiB and kaiC (Ishiura et al. 1998).

From these results it is clear that KaiA is a positive enhancer of kaiB and kaiC. Expression of it increases the levels of kaiB and kaiC. On the other hand, KaiC is a negative repressor of the kaiBC loop. KaiC inhibits its own transcription and as kaiB shares the same promoter this is also down regulated.

The exact role of the 3 kai proteins is unclear. None of them contain any obvious DNA binding motifs so it may be argued that they are not transcription factors. However all 3 proteins will bind together, no only in vitro, but also in yeast two-hybrid assays. This interaction though has a physiological significance as an interaction between a mutant KaiA protein and a normal KaiB protein produces a long-period phenotype (Iwasaki et al. 1999)  

The cyanobacteria clock. Proteins with positive actions are in yellow, negative components are in blue. Protein-protein interactions are indicated with dotted black arrows. Green arrows indicate a positive effect on some process or promoter, and red lines indicate an inhibitory action. 

(From Harmer 2001)

There may be other proteins that have a fringe role in the circadian clock of Cyanobacteria. SasA, a histidine kinase interacts with KaiC. Disruption of this protein affects both the amplitude and period of the circadian rhythm.

                       

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