Circadian programming in cyanobacteria

Prokaryotic cyanobacteria express robust circadian (daily) rhythms under th e control of a timing mechanism that is independent of the cell division cy cle. This biological clock orchestrates global regulation of gene expressio n. Competition experiments demonstrate that fitness is enhanced when the ci rcadian period is consonant with the period of the environmental cycle. Mul tational analyses have identified three clock genes in the organism, one of which is related to DNA recombinases and helicases. We propose a new model for the Corp 'clockwork' that implicates rhythmic changes in the status of the chromosome that underly the rhythms of gene expression.