SPECIFICITY OF DNA RECOGNITION IN THE NUCLEOPROTEIN COMPLEX FOR SITE-SPECIFIC RECOMBINATION BY TN21 RESOLVASE

Resolvases from Tn3-like transposons catalyse site-specific recombinat ion at res sites. Each res site has 3 binding sites for resolvase, I, II, and III. The res sites in Tn3 and Tn21 have similar structures at I and II but they differ at III. Mutagenesis of the Tn21 res site show ed that sub-site III is essential for recombination though the sequenc es in III that are recognized by Tn21 resolvase are positioned differe ntly from the equivalent sequences in the Tn3 site. The deletion of II I caused a 1,000-fold drop in the rate of recombination. But other mut ations at III, changing 3 or 4 consecutive base pairs, caused only 1.5 - to 4-fold decreases in rate, even when the mutations were in target sequences for this helix-turn-helix protein. The reason why Tn21 resol vase has similar activities at a number of different DNA sequences may be due to the multiplicity of protein-protein and protein-DNA interac tions in its recombinogenic complex. This lack of precision may be a g eneral feature of nucleoprotein complexes.