The beta recombinase of plasmid pSM19035 binds to two adjacent sites, making different contacts at each of them.

The beta recombinase from plasmid pSM19035 catalyzes intramolecular site-specific recombination between two directly or inversely oriented six sites in the presence of a chromatin-associated protein (Hbsu, HU or HMG-1). The six site is a DNA segment containing two binding sites (I and II) for beta protein dimers. We show that beta recombinase binds sequentially to both sites, having a different affinity for each one. Hydroxyl radical footprints show a different protection pattern at each site. Positions critical for beta protein binding have been identified by methylation interference and missing nucleoside assays. The results indicate that the protein recognizes each site in a different way. Comparison of the beta protein recombination site with that of DNA resolvases and DNA invertases of the Tn3 family, to which it belongs, shows that these sequences can be divided into two regions. One corresponds to the crossover point and is similar for all recombinases of the family. The other region differs in the different subfamilies and seems to have an architectural role in aligning the crossover sites at the synaptic complex. The different ways to assemble this complex could explain why each system leads to a particular recombination event: DNA resolution (resolvases), inversion (invertases) or both (beta recombinase).