Abstract
Cyanobacteria are the first prokaryotes reported to show circadian rhythmicity, which is regulated by a cluster of three genes: kaiA, kaiB, and kaiC. Phylogenetic analysis of the kaiBC cluster in filamentous cyanobacteria of the family Nostocaceae including Nodularia spumigena and Nostoc linckia from Arubotaim Cave, Mt. Sedom, Israel, indicated that this cluster has experienced multiple lateral transfers. The transfers have occurred in different periods of the species' evolution. The data obtained suggest that lateral transfers of the circadian clock cluster in filamentous cyanobacteria have been common and might have adaptive significance.