Question

Some photosynthetic Cyanobacteria can fix N2 to NH4 + (using the enzyme Nitrogenase) when they are otherwise starved for nitrogen. They sense nitrogen starvation by sensing an excess of the amino acid Glutamate (Glu) within the cell. If there is adequate nitrogen, much of the Glutamate is converted to Glutamine (Gln), so that the [Glutamate] remains low.

A. SKETCH the photosynthesis system in a typical aerobic Cyanobacterium. Be sure to show how O2 is involved, and how the three main forms of energy are generated.

B. When the [Glu] gets too high, the genes for Photosystem II are shut off, and the Nitrogenase genes are turned on. Assume that the Nitrogenase genes are under Negative control, and the PS-II genes are under Positive control. Make sketches showing how [Glu] affects both of these operons. Then explain each regulatory scheme in words. Be sure to label the regulatory proteins (‘A’ for activator and ‘R’ for repressor protein).

C. On your diagram from part (A), show and explain how turning off the genes for Photosystem II will affect the photosynthetic electron flow in this Cyanobacterium? Why is it important to turn off Photosystem II before the cells produce Nitrogenase?

D. Nitrogen fixation by Cyanobacteria (even with the normal regulation, as described above) is a lot more effective if the Cyanobacteria are grown in co-culture with nitrifying bacteria. Explain why the co-culture with nitrifiers helps with the process of nitrogen fixation.

Answer 1

A.

B. Heterocysts (nitrogen fixing cell) provide the N2 source required for the metabolism of vegetative cells. (GLU) transports from vegetative cells to heterocysts(nitrogen fixing cell) and the fixed N2 in the form of (GLN) with an additional N2 atom than (GLU) with the same Carbon(C) skeleton shuttles back to vegetative cells as the N2 source for growth.Hence, the entire filament can grow by incorporating fixed N2 provided in heterocysts cell. There is an exchange of 3 metabolites (glutamine, glutamate, and sucrose).When the [Glu] gets too high, the genes for Photosystem II are shut off, and the Nitrogenase genes are turned on,so when Nitrogenase genes are under Negative control, and the PS-II genes are under Positive control.The required amount of N2 will not be available to fullfil the starvation.

C. Turning off the genes for Photosystem II will affect the photosynthetic electron flow in this Cyanobacterium.The PS II is the system that increases the level of amino acid Glutamate. By shutting down the gene for PS II the glutamate is converted to glutamine,which produce nitrogen.

D. Nitrogen fixation by Cyanobacteria is a lot more effective if the Cyanobacteria are grown in co-culture with nitrifying bacteria,because the high oxygen stress cause the proteolysis of nitrogenase ,suppress the nitrogenase systhesis.