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

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 α and σ 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 β strand. The N-terminal dimerization domain has an unusual mode of dimerization; helices α1 and α2 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 β strands. Mutational analysis of key leucine residues in helix α1 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 β strand may act as pivot points, facilitating the conformational changes of Mor, and the DNA may be bent.