Polynucleotidyl transfer reactions in site-specific DNA recombination

Site-specific DNA rearrangement reactions are widespread among organisms. They are used, for example, by vertebrates to boost immune response diversity, and in turn by parasitic organisms to evade the host immune system by surface antigen switching. Parasitic genetic elements ubiquitous to most organisms invade new host genomic sites by a variety of types of site-specific recombination. Polynucleotidyl transfer reactions are central to these DNA recombination reactions. The recombinase of each reaction system that 'catalyses' such chemical reactions at specific DNA sites are apparently designed to accomplish unique DNA geometrical specificity, or delicate control over the extent or direction of the reaction, with the sacrifice of protein turnover. Here we discuss our current understanding of several issues that relate to the polynucleotidyl transfer steps in several of the better studied site-specific recombination reactions.