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J Mol Biol. 2009 Feb 20;386(2):554-68. doi: 10.1016/j.jmb.2008.12.057. Epub 2008 Dec 30.

A novel fold in the TraI relaxase-helicase c-terminal domain is essential for conjugative DNA transfer.

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Department of Chemistry, University of North Carolina at Chapel Hill, 27599-3290, USA.


TraI relaxase-helicase is the central catalytic component of the multiprotein relaxosome complex responsible for conjugative DNA transfer (CDT) between bacterial cells. CDT is a primary mechanism for the lateral propagation of microbial genetic material, including the spread of antibiotic resistance genes. The 2.4-A resolution crystal structure of the C-terminal domain of the multifunctional Escherichia coli F (fertility) plasmid TraI protein is presented, and specific structural regions essential for CDT are identified. The crystal structure reveals a novel fold composed of a 28-residue N-terminal alpha-domain connected by a proline-rich loop to a compact alpha/beta-domain. Both the globular nature of the alpha/beta-domain and the presence as well as rigidity of the proline-rich loop are required for DNA transfer and single-stranded DNA binding. Taken together, these data establish the specific structural features of this noncatalytic domain that are essential to DNA conjugation.

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