ESCHERICHIA-COLI DNA TOPOISOMERASE-I AND SUPPRESSION OF KILLING BY TN5 TRANSPOSASE OVERPRODUCTION - TOPOISOMERASE-I MODULATES TN5 TRANSPOSITION

Tn5 transposase (Tnp) overproduction is lethal to Escherichia coli. Th e overproduction causes cell filamentation and abnormal chromosome seg regation. Here we present three lines of evidence strongly suggesting that Tnp overproduction killing is due to titration of topoisomerase I . First, a suppressor mutation of transposase overproduction killing, stkD10, is localized in topA (the gene for topoisomerase I). The stkD1 0 mutant has the following characteristics: first, it has an increased abundance of topoisomerase I protein, the topoisomerase I is defectiv e for the DNA relaxation activity, and DNA gyrase activity is reduced; second, the suppressor phenotype of a second mutation localized in rp oH, stkA14 (H. Yigit and W. S. Reznikoff, J. Bacteriol. 179:1704-1713, 1997), can be explained by an increase in topA expression; and third, overexpression of wild-type topA partially suppresses the killing. Fi nally, topoisomerase I was found to enhance Tn5 transposition up to 30 -fold in vivo.