Some photosynthetic Cyanobacteria can fix N2 to NH4 + (using theenzyme Nitrogenase) when they are otherwise starved for nitrogen.They sense nitrogen starvation by sensing an excess of the aminoacid Glutamate (Glu) within the cell. If there is adequatenitrogen, much of the Glutamate is converted to Glutamine (Gln), sothat the [Glutamate] remains low.
A. SKETCH the photosynthesis system in a typical aerobicCyanobacterium. Be sure to show how O2 is involved, and how thethree main forms of energy are generated.
B. When the [Glu] gets too high, the genes for Photosystem IIare shut off, and the Nitrogenase genes are turned on. Assume thatthe Nitrogenase genes are under Negative control, and the PS-IIgenes are under Positive control. Make sketches showing how [Glu]affects both of these operons. Then explain each regulatory schemein words. Be sure to label the regulatory proteins (‘A’ foractivator and ‘R’ for repressor protein).
C. On your diagram from part (A), show and explain how turningoff the genes for Photosystem II will affect the photosyntheticelectron flow in this Cyanobacterium? Why is it important to turnoff Photosystem II before the cells produce Nitrogenase?
D. Nitrogen fixation by Cyanobacteria (even with the normalregulation, as described above) is a lot more effective if theCyanobacteria are grown in co-culture with nitrifying bacteria.Explain why the co-culture with nitrifiers helps with the processof nitrogen fixation.
