Insertion of the tetracycline resistance translocation unit Tn10 in thelac operon ofEscherichia coli K12

Abstract

The majority of Tn10 insertions in thelacZ gene ofEscherichia coli occurred in a small region of the promoter distal part of the gene. The resulting mutations were polar onlacY and reverted to Lac⁺ at a frequency of 10^-8. None of the revertants were Tc⁴. Furthermore Lac⁺ Tc^r revertants could not be selected directly. Relief of polarity revertants of thelacZ::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 Tc^r 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.