The beta recombinase from the broad host range Grampositive plasmid pSM19035 catalyzes intramolecular site-specific recombination between two directly or inversely oriented recombination sites in the presence of a chromatin-associated protein (Hbsu). The recombination site had been localized to a 447 bp DNA segment from pSM19035. This segment includes a 90 bp region that contains two adjacent binding sites (I and II) for beta protein dimers. Using in vitro recombination assays, we show that this 90 bp region is necessary and sufficient for beta protein-mediated recombination; this defines the six site as the region required for beta protein binding. The point of crossing over has been localized to the center of site I. Hbsu has a strong binding affinity for an unknown site located within the 447 bp segment containing the six site. We discuss the possibility that Hbsu recognizes an altered DNA structure, rather than a specific sequence, generated in the synaptic complex.