Setting of the Circadian N(2)-Fixing Rhythm of the Prokaryotic Synechococcus sp. RF-1 while Its nif Gene Is Repressed

The N₂-fixing activity of the prokaryotic Synechococcus sp. RF-1 was repressed in the presence of nitrate. When the cultures in nitrate-containing medium were exposed to diurnal light-dark cycles, an endogenous circadian N₂-fixing rhythm developed after the cells were transferred to nitrate-free medium and incubated in continuous light. The N₂-fixing phase of the rhythm coincided with the dark phase of the light-dark cycles that were imposed when the cells were in nitrate-containing medium. The results indicate that after the endogenous N₂-fixing rhythm has been set, it can be kept latent for at least 38 hours before first manifesting itself.