STRUCTURAL FEATURES OF THE PLASMID PMV158-ENCODED TRANSCRIPTIONAL REPRESSOR COPG, A PROTEIN SHARING SIMILARITIES WITH BOTH HELIX-TURN-HELIXAND BETA-SHEET DNA-BINDING PROTEINS

The small transcriptional repressor CopG protein (45 amino acids) enco ded by the streptococcal plasmid pMV158 was purified to near homogenei ty. Gel filtration chromatography and analytical ultracentrifugation s howed that the native protein is a spherical dimer of identical subuni ts. Circular dichroism measurements of CopG indicated a consensus aver age content of more than 50% alpha-helix and 10-35% beta-strand and tu rns, which is compatible with the predicted secondary structure of the protein. CopG exhibited a prolonged intracellular half-life, but dele tions in regions other than the C-terminal affected the global structu re of the protein, severely reducing the half-lives of the CopG varian ts. This indicates that CopG has a compact structure, perhaps constitu ted by a single domain. Molecular modeling of CopG showed a good fitti ng between the helix-turn-helix motifs of well-known repressor protein s and a bihelical unit of CopG, However, modeling of CopG with ribbon- helix-helix class of DNA binding proteins also exhibited an excellent fit. Eleven out of the 12 replicons belonging to the pMV158 plasmid fa mily could also encode Cop proteins, which share features with both he lix-turn-helix and beta-sheet DNA binding proteins. Proteins 32:248-26 1, 1998. (C) 1998 Wiley-Liss, Inc.