X-ray studies conducted at SSRL have shed light on a sword-and-shield type of defense used by bacteria to protect themselves from viral attacks. This new knowledge could prove significant to medical research on human cancers caused by a similar defense mechanism gone awry.

Researchers from the City of Hope cancer research and treatment center in Duarte, California, determined the crystal structure of the protein that controls this defense system in bacteria called Bacillus caldolyticus. Unless stopped, viral DNA slips into bacterial DNA, where it gets copied many times over, and then destroys its host. To protect bacterial cells, the control protein ensures the proper ratio between two enzymes, the "swords" and the "shields." The sword enzyme slashes invading viral DNA into useless pieces. The shield enzyme adds a protective layer to bacterial DNA, so the sword will not cut its master. Too few shields lead to bacterial cell death, and too many shields protect the viral DNA as well.

The crystal structure of the control protein uncovers the presence of a helix-turn-helix (HTH) motif which has the potential to bend B-DNA. The structural study also revealed amino acid residues that are most likely involved in the DNA interaction. The proposed model suggests that the protein adjusts the levels of the defense enzymes (swords and shields) by sliding along the bacterial DNA, bending and changing its shape to turn on or off the respective genes.

To learn more about this research see the full scientific highlight at:
http://www-ssrl.slac.stanford.edu/ research/highlights_archive/cbcli.html

Sawaya, M. R. Zhu, Z., Mersha, F., Chan, S-h., Dabur, R., Xu, S-y., Balendiran, G. K., Crystal structure of the restriction-modification system control element C.BclI and mapping of its binding site. Structure 13:1837-1847, (2005).