Site-specific recombination and topoisomerization by Tn21resolvase: role of metal ions

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Abstract
The resolvase from the transposon Tn 21 catalyses site-specific recombination between the two res sites on its DNA substrate both in the absence and presence of Mg 2+ ions. This contrasts with reports on the resolvase from gamma-delta (Tn 1000 ) and on other recombinational proteins that are homologous to Tn 21 resolvase but which need Mg 2+ for their activity. Magnesium ions could enhance the activity of Tn 21 resolvase, as did a number of other cations but some metal ions such as Ni 2+ inhibit recombination. The metal ions are not directly involved in the catalytic process but probably affect recombination by altering the conformation of the DNA. Tn 21 resolvase relaxes its DNA substrate in the presence and in the absence of Mg 2+ , and also in ionic conditions that inhibit recombination. Hence, the topoisomerization reflects an activity of resolvase that is distinct from recombination. However, the two activities are functions of the same DNA-protein complex. The complex contains about 6 molecules of the resolvase dimer per molecule of DNA.