The Resolvase Protein of the Transposon γδ

The transposable elements γδ and Tn3 encode an efficient site-specific recombination system essential in the normal transposition process. Transposition of these elements appears to occur in a two-step process. An element-encoded protein, the transposase, is involved in the initial step that leads to a fused replicon or cointegrates. In this structure, two copies of the element are present in the same orientation at the junctions between the donor and target replicons. The resolvase protein of γδ or Tn3 catalyzes a site-specific recombination reaction at a particular site, res, located within the element. This recombination leads to conversion of the cointegrate to the two end products of transposition, the donor replicon, and the target replicon, now containing a copy of the element. This second step in the transposition of Tn3-1ike elements, termed resolution, is demonstrated in a purified, in vitro system. The site at which recombination occurs is identified through in vivo and in vitro techniques. This chapter outlines the methods currently used to purify and assay the resolvase protein of γδ. The closely related protein from Tn3 is investigated elsewhere, and comparisons of the properties of these two enzymes is enlightening.