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J. Biol. Chem., Vol. 280, Issue 31, 28683-28691, August 5, 2005

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The Unstructured N-terminal Tail of ParG Modulates Assembly of a Quaternary Nucleoprotein Complex in Transcription Repression^*

Emma Carmelo, Daniela Barillà, Alexander P. Golovanov, Lu-Yun Lian, Andrew Derome, and Finbarr Hayes

From the Faculty of Life Sciences, The University of Manchester, Manchester M60 1QD, United Kingdom

ParG is the prototype of a group of small (<10 kDa) proteins involved in accurate plasmid segregation. The protein is a dimeric DNA binding factor, which consists of symmetric paired C-terminal domains that interleave into a ribbon-helix-helix fold that is crucial for the interaction with DNA, and unstructured N-terminal domains of previously unknown function. Here the ParG protein is shown to be a transcriptional repressor of the parFG genes. The protein assembles on its operator site initially as a tetramer (dimer of dimers) and, at elevated protein concentrations, as a pair of tetramers. Progressive deletion of the mobile N-terminal tails concomitantly decreased transcriptional repression by ParG and perturbed the DNA binding kinetics of the protein. The flexible tails are not necessary for ParG dimerization but instead modulate the organization of a higher order nucleoprotein complex that is crucial for proper transcriptional repression. This is achieved by transient associations between the flexible and folded domains in complex with the target DNA. Numerous ParG homologs encoded by plasmids of Gram-negative bacteria similarly are predicted to possess N-terminal disordered tails, suggesting that this is a common feature of partition operon autoregulation. The results provide new insights into the role of natively unfolded domains in protein function, the molecular mechanisms of transcription regulation, and the control of plasmid segregation.

Received for publication, February 1, 2005 , and in revised form, May 23, 2005.

^* This work was supported by grants from The Wellcome Trust and the Biotechnology and Biological Sciences Research Council (to F. H.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

A Medical Research Council New Investigator.

To whom correspondence should be addressed: Faculty of Life Sciences, The University of Manchester, P. O. Box 88, Sackville St., Manchester M60 1QD, UK. Tel.: 44-161-3068934; Fax: 44-161-2360409; E-mail: finbarr.hayes{at}manchester.ac.uk.

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