EXPRESSION OF A GENE-CLUSTER KAIABC AS A CIRCADIAN FEEDBACK PROCESS IN CYANOBACTERIA

Cyanobacteria are the simplest organisms known to have a circadian clo ck. A circadian crock gene cluster kaiABC was cloned from the cyanobac terium Synechococcus. Nineteen clock mutations were mapped to the thre e kai genes. Promoter activities upstream of the kaiA and kaiB genes s howed circadian rhythms of expression, and both kaiA and kaiBC messeng er RNAs displayed circadian cycling. inactivation of any single kai ge ne abolished these rhythms and reduced kaiBC-promoter activity. Contin uous kaiC overexpression repressed the kaiBC promoter, whereas kaiA ov erexpression enhanced it. Temporal kaiC overexpression reset the phase of the rhythms. Thus, a negative feedback control of kaiC expression by KaiC generates a circadian oscillation in cyanobacteria, and KaiA s ustains the oscillation by enhancing kaiC expression.