Synapsis and strand exchange in the resolution and DNA inversion reactions catalysed by the beta recombinase

In the presence of a sequence-independent chromatin-associated protein, such as Hbsu or HMGB, the β recombinase catalyses resolution between two directly oriented recombination sites (six sites) and both resolution and DNA inversion between two inversely oriented six sites. Assembly of the synaptic complex requires binding of the β recombinase to the six sites and the presence of Hbsu. Whether resolution or inversion will take place depends on the relative orientation of the two six sites, the level of DNA supercoiling and the amounts of Hbsu. In this work, the topologies of the products of the resolution and inversion reactions were analysed. The resolution reaction generated mainly singly catenated DNA circles, while DNA inversion gave rise to unknotted circles and small amounts of DNA molecules containing 3- or 5-noded knots. In spite of the distinctive features of the β system, the topology of synapsis and strand exchange during the resolution reaction is similar to that of Tn3 and γδ resolvases. The ability of the β recombinase to catalyse both inversion and resolution reactions probably reflects different possible architectures of the synaptic complex, which to a large extent depends on Hbsu.