Escherichia coli DNA topoisomerase I copurifies with Tn5 transposase, and Tn5 transposase inhibits topoisomerase I

Tn5 transposase (Tnp) overproduction is lethal to Escherichia coli. Genetic evidence suggested that this killing involves titration of E. coil topoiso merase I (Topo I). Here, we present biochemical evidence that supports this model. Tn5 Tnp copurifies with Topo I while nonkilling derivatives of Tnp, Delta 37Tnp and Delta 55Tnp (Inhibitor [Inh]), show reduced affinity or no affinity, respectively, for Topo I. In agreement with these results, the p resence of Tnp, but not Delta 37 or Inh derivatives of Tnp, inhibits the DN A relaxation activity of Topo I in vivo as well as in vitro. Other proteins , including RNA polymerase, are also found to copurify with Tnp, For RNA po lymerase, reduced copurification with Tnp is observed in extracts from a to pA mutant strain, suggesting that RNA polymerase interacts with Topo I and not Tnp.