Multiple IS10 Rearrangements in Escherichia Coli

We have investigated the occurrence of multiple transposon-promoted chromosomal rearrangements in Escherichia coli K12 strains containing transposon Tn10. We show that a single Tn10 element, with its two closely spaced insertion sequence (IS10) elements, frequently gives rise to complex rearrangements that can be accounted for as the sum of two "classical" IS10 events. Using a strain containing differentially marked Tn10 elements at widely separated locations, we have investigated the possibility that IS10-promoted rearrangements occur in cell-wide "bursts", as expected if cells could occasionally undergo brief periods when all IS10 transposition events were activated, interspersed with longer periods of relative quiescence. We find no evidence for strong (greater than 60-fold), periodic cell-wide activation under our experimental conditions. The sensitivity of this experiment has been evaluated using an expression for the accumulation of double mutations in populations with heterogeneous, fluctuating mutation rates (see Appendix). We discuss several mechanisms by which two closely linked IS10 elements could undergo coupled double events without cell-wide activation: local activation of small chromosomal regions, periodic bursts of synthesis of cis-acting transposase protein, and/or a propensity for elements that have actually engaged in one rearrangement event to initiate a second successive event immediately thereafter. We favor the last possibility.