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J. Biol. Chem., Vol. 279, Issue 16, 16581-16590, April 16, 2004

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Crystal Structure of the Mor Protein of Bacteriophage Mu, a Member of the Mor/C Family of Transcription Activators^*

Muthiah Kumaraswami, Martha M. Howe, and Hee-Won Park¶||

From the Department of Molecular Sciences, University of Tennessee Health Science Center, Memphis, Tennessee 38163 and the ^¶Department of Structural Biology, St. Jude Children's Research Hospital, Memphis, Tennessee 38105

Transcription from the middle promoter, P_m, of bacteriophage Mu requires the phage-encoded activator protein Mor and bacterial RNA polymerase. Mor is a sequence-specific DNA-binding protein that mediates transcription activation through its interactions with the C-terminal domains of the and subunits of bacterial RNA polymerase. Here we present the first structure for a member of the Mor/C family of transcription activators, the crystal structure of Mor to 2.2-Å resolution. Each monomer of the Mor dimer is composed of two domains, the N-terminal dimerization domain and C-terminal DNA-binding domain, which are connected by a linker containing a strand. The N-terminal dimerization domain has an unusual mode of dimerization; helices 1 and 2 of both monomers are intertwined to form a four-helix bundle, generating a hydrophobic core that is further stabilized by antiparallel interactions between the two strands. Mutational analysis of key leucine residues in helix 1 demonstrated a role for this hydrophobic core in protein solubility and function. The C-terminal domain has a classical helix-turn-helix DNA-binding motif that is located at opposite ends of the elongated dimer. Since the distance between the two helix-turn-helix motifs is too great to allow binding to two adjacent major grooves of the 16-bp Mor-binding site, we propose that conformational changes in the protein and DNA will be required for Mor to interact with the DNA. The highly conserved glycines flanking the strand may act as pivot points, facilitating the conformational changes of Mor, and the DNA may be bent.

Received for publication, December 11, 2003 , and in revised form, January 9, 2004.

The atomic coordinates and structure factors (code 1RR7) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/).

^* This work was supported in part by National Science Foundation Grant MCB-0318108, University of Tennessee Center of Excellence in Structural Biology grants (to M. M. H. and H.-W. P), Cancer Center Support Grant CA 21765, and American Lebanese Syrian Associated Charities grants (to H.-W. P.). 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.

Recipient of a University of Tennessee Van Vleet Professorship.

^|| To whom correspondence should be addressed: Dept. of Structural Biology, St. Jude Children's Research Hospital, 332 N. Lauderdale, Memphis, TN 38105. Tel.: 901-495-3838; Fax: 901-495-3032; E-mail: hee-won.park{at}stjude.org.

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