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The key to the lock that controls nitrogen fixation

November 04, 2004

"Bacteria that fix nitrogen only do so when they sense that there is very little nitrogen available in their environment," says Professor Ray Dixon (Project Leader at the JIC. "Normally the genes for nitrogen fixation are locked off and only unlocked and used when nitrogen levels in the environment fall. We have discovered a key piece of biochemistry that allows us to better understand how the lock operates and so may allow us to alter how it works".

The bacterium Azotobacter vinelandii is able to fix atmospheric nitrogen when available nitrogen in its environment falls below a threshold level. Nitrogen fixation requires a great deal of energy and so the genes that carry out nitrogen fixation (so called nif genes) are tightly regulated and switched off when not required.

The nif genes are regulated by the action of two proteins, called NifL and NifA. NifA stimulates the activity of nif genes, while NifL normally binds to NifA and renders it inactive. Thus whether the nif genes are active or not depends on the interaction between these two proteins. Both proteins are sensitive to biochemical signals that occur in the bacterial cell when conditions are right for nitrogen fixation. The proteins' physical shape and structure alters in response to these signals and this affects their ability to bind to one another. The result is that, when conditions are right for nitrogen fixation, NifA is released from the grip of NifL and is then able to stimulate the activity of the nif genes and so switches on nitrogen fixation by the cell.

The latest research has identified a single amino acid change in the NifL protein that prevents the molecule releasing NifA when the appropriate signals are present [2]. This gives the scientists an important clue about the key processes which operate the lock that controls nitrogen fixation.

The discovery will be reported in the international science journal Proceedings of the National Academy of Sciences US, and is available on line in the PNAS Online Early Edition [3].

John Innes Centre

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