Insertion of the tetracycline resistance translocation unit Tn10 in the lac operon of Escherichia coli K12

Abstract

The majority of TN10 insertions in the lacZ gene of Escherichia coli occurred in a small region of the promoter distal part of the gene. The resulting mutations were polar on lacY and reverted to Lac+ at a frequency of 10(-8). None of the revertants were Tcr. Furthermore Lac+ Tcr revertants could not be selected directly. Relief of polarity revertants of the lacZ::Tn10 mutants were formed at a frequency of 10(-5) - 10(-4). Most resulted from a deletion event internal to the transposon which removed the Tcr genes and the putative transcription terminator. It is postulated that a fragment of Tn10 remains at the original insertion point to cause a revertible Lac- mutation.