Abstract
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.
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