The circadian clock and darkness control natural competence in cyanobacteria

Natural genetic competence-based transformation contributed to the evolution of prokaryotes, including the cyanobacterial phylum that established oxygenic photosynthesis. The cyanobacterium Synechococcus elongatus is noted both as a model system for analyzing a prokaryotic circadian clock and for its facile, but poorly understood, natural competence. Here a genome-wide screen aimed at determining the genetic basis of competence in cyanobacteria identified all genes required for natural transformation in S. elongatus, including conserved Type IV pilus, competence-associated, and newly described genes, and revealed that the circadian clock controls the process. The findings uncover a daily program that determines the state of competence in S. elongatus and adapts to seasonal changes of day-length. Pilus biogenesis occurs daily in the morning, but competence is maximal upon the coincidence of circadian dusk and the onset of darkness. As in heterotrophic bacteria, where natural competence is conditionally regulated by nutritional or other stress, cyanobacterial competence is conditional and is tied to the daily cycle set by the cells most critical nutritional source, the Sun.