MULTIPLE IS10 REARRANGEMENTS IN ESCHERICHIA-COLI

The occurrence of multiple transposon-promoted chromosomal rearrangements in E. coli K12 strains containing transposon Tn10 was investigated. A single Tn10 element, with its 2 closely spaced insertion sequence (IS10) elements, frequently gives rise to complex rearrangements that can be accounted for as the sum of 2 classical IS10 events. Using a strain containing differentially marked Tn10 elements at widely separated locations, the possibility was investigated 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. No evidence was found for strong (> 60-fold), periodic cell-wide activation under the experimental conditions. The sensitivity of this experiment was evaluated using an expression for the accumulation of double mutations in populations with heterogeneous, fluctuating mutation rates. Several mechanisms by which 2 closely linked IS10 elements could undergo coupled double events without cell-wide activation are discussed: local activation of small chromosomal regions, periodic bursts of synthesis of cis-acting transposase protein and/or a propensity for elements that actually engaged in 1 rearrangement event to initiate a 2nd successive event immediately thereafter. The last possibility is favored.