RECIPROCAL LIGHT-DARK TRANSCRIPTIONAL CONTROL OF NIF AND RBC EXPRESSION AND LIGHT-DEPENDENT POSTTRANSLATIONAL CONTROL OF NITROGENASE ACTIVITY IN SYNECHOCOCCUS SP STRAIN RF-1

Synechococcus sp. strain RF-1 exhibits a circadian rhythm of N-2 fixat ion when cells are grown under a light-dark cycle, with nitrogenase ac tivity observed only during the dark period. This dark-dependent activ ity correlated with nif gene transcription in strain RF-1. By using an tibodies against dinitrogenase reductase (the Fe protein of the nitrog enase complex), it was found that there was a distinct shift in the mo bility of this protein on sodium dodecyl sulfate gels during the light -dark cycle. The Fe protein was present only when cells were incubated in the dark Upon illumination, there was a conversion of all Fe prote in to a modified form, after which it rapidly disappeared from extract s. These studies indicated that all nitrogenase activity present durin g the dark cycle resulted from de novo synthesis of nitrogenase. Upon entering the light phase, cells appeared to quickly degrade the modifi ed form of Fe protein, perhaps as a result of activating or inducing a protease. By contrast, transcription of the rbcL gene, which encodes the catalytic subunit of the key enzyme of CO2 fixation (a light-depen dent process), was enhanced in the light.