Plant disease proteins caught in the act

Phytopathogenic bacteria, as the name suggests, wreak havoc on many important crops, including rice, pepper, citrus and cotton. They inject a cocktail of virulence factors into the plant. Understanding how these factors recognize and bind to the host's DNA will reveal how bacteria grapple with their host DNA.

Now, a team of researchers, including Magdy Mahfouz from King Abdullah University of Science and Technology (KAUST) in Thuwal, Saudi Arabia, has captured the crystal structure of one such type of virulence factor, known as transcription activator-like effectors (TALES), in both free form and bound to plant DNA.

The unbound TALE forms a helix-shaped structure with a steep incline. The protein molecules can link together, forming a superhelical structure. The TALES-DNA complex is very similar in structure to the free form, but after the first 22 amino acids of TALES form a subtle, yet significant, modification that compresses the helix.

The TALE wraps around the major groove of the DNA double helix. A ridge of positively charged amino acids help the protein bind to the negatively charged DNA phosphate groups. Another binding mechanism involves a central domain made up of tandem repeats of the protein molecule. Each repeat recognizes a DNA base pair through amino acids at positions 12 and 13. Residue 12 bolsters TALE's shape, while residue 13 directly binds to DNA.

This new data will help biotechnologists design proteins that mimic TALES but which can be used against the bacteria to stop the rot and save yield.

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