The structure of plasmid-encoded transcriptional repressor CopG unligandedand bound to its operator

The structure of the 45 amino acid transcriptional repressor, CopG, has bee n solved unliganded and bound to its target operator DNA, The protein, enco ded by the promiscuous streptococcal plasmid pMV158, is involved in the con trol of plasmid copy number. The structure of this protein repressor, which is the shortest reported to date and the first isolated from a plasmid, ha s a homodimeric ribbon-helix-helix arrangement. It is the prototype for a f amily of homologous plasmid repressors. CopG cooperatively associates, comp letely protecting several turns on one face of the double helix in both dir ections from a 13-bp pseudosymmetric primary DNA recognition element. In th e complex structure, one protein tetramer binds at one face of a 19-bp olig onucleotide, containing the pseudosymmetric element, with two beta-ribbons inserted into the major groove. The DNA is bent 60 degrees by compression o f both major and minor grooves. The protein dimer displays topological simi larity to Are and MetJ repressors. Nevertheless, the functional tetramer ha s a unique structure with the two vicinal recognition ribbon elements at a short distance, thus inducing strong DNA bend. Further structural resemblan ce is found with helix-turn-helix regions of unrelated DNA-binding proteins . In contrast to these, however, the bi-helical region of CopG has a role i n oligomerization instead of DNA recognition. This observation unveils an e volutionary link between ribbon-helix-helix and helix-turn-helix proteins.