Crystal structure of omega transcriptional repressor encoded by Streptococcus pyogenes plasmid pSM19035 at 1.5 A resolution.

The 71 amino acid residue omega protein encoded by the Streptococcus pyogenes non-conjugative plasmid pSM19035 is a transcriptional repressor that regulates expression of genes for copy number control and stable maintenance of plasmids. The crystal structure of omega protein has been determined by multiple isomorphous replacement, including anomalous scattering and refined to an R-factor of 21.1 % (R(free)=23.2 %) at 1.5 A resolution. Two monomers related by a non-crystallographic 2-fold axis form a homodimer that occupies the asymmetric unit. Each polypeptide chain is folded into two alpha-helices and one beta-strand forming an antiparallel beta-ribbon in the homodimer. The N-terminal regions (1-23 and 1-22 in subunits I and II, respectively) are not defined in the electron density due to proteolysis of the N-terminal 20 amino acid residues during crystallisation and partial disorder. The omega protein belongs to the structural superfamily of MetJ/Arc repressors featuring a ribbon-helix-helix DNA-binding motif with the beta-ribbon located in and recognizing the major groove of operator DNA; according to a modelled omega protein-DNA complex, residues Arg31 and Arg31' on the beta-ribbon are in positions to interact with a nucleobase, especially guanine.