THE TRANSACTIVATION REGION OF THE FIS PROTEIN THAT CONTROLS SITE-SPECIFIC DNA INVERSION CONTAINS EXTENDED MOBILE BETA-HAIRPIN ARMS

The Fis protein regulates site-specific DNA inversion catalyzed by a f amily of DNA invertases when bound to a cis-acting recombinational enh ancer, As is often found for transactivation domains, previous crystal structures have failed to resolve the conformation of the N-terminal inversion activation region within the Fis dimer. A new crystal form o f a mutant Fis protein now reveals that the activation region contains two beta-hairpin arms that protrude over 20 Angstrom from the protein core. Saturation mutagenesis identified the regulatory and structural ly important amino acids. The most critical activating residues are lo cated near the tips of the beta-arms. Disulfide cross-linking between the beta-arms demonstrated that they are highly flexible in solution a nd that efficient inversion activation can occur when the beta-arms ar e covalently linked together. The emerging picture for this regulatory motif is that contacts with the recombinase at the tip of the mobile beta-arms activate the DNA invertase in the context of an invertasome complex.