THE F PLASMID TRAY GENE-PRODUCT BINDS DNA AS A MONOMER OR A DIMER - STRUCTURAL AND FUNCTIONAL IMPLICATIONS

The F factor traY gene product (TraYp) is a site-specific DNA-binding protein involved in initiation of DNA transfer during bacterial conjug ation. The sequence of TraYp exhibits a unique direct-repeat structure predicted to have a ribbon-helix-helix DNA-binding motif in each repe at unit. The stoichiometry of TraYp binding to DNA was determined to f urther support the hypothesis that TraYp is a member of the ribbon-hel ix-helix family of DNA-binding proteins. A glutathione-S-transferase-t raY fusion protein was purified and shown to possess almost wild-type DNA-binding activity. DNA-binding experiments were performed in which the DNA ligand was incubated with either the fusion protein, the wild- type protein, or both. The results indicate that TraYp can bind DNA as a monomer or a dimer. Thus a TraYp monomer folds into a stable three- dimensional structure similar to that of a dimer of the ribbon-helix-h elix proteins Are or Mnt. A homology model of a TraYp monomer has been constructed using the co-crystal structure of Are bound to DNA as a t emplate to provide additional support for this conclusion. In addition , we have shown that an origin of the transfer-deletion mutant lacking approximately half of the TraYp-binding site can only be bound by a m onomer of TraYp. The functional implications of this result are discus sed.