[Nature of the plasmids determining the unstable inheritance of transposon Tn9 in Escherichia coli K-12].

Unstable inheritance of transposon Tn9 in the Escherichia coli strain KS7201 had been connected with its integration into a certain bacterial chromosome site (attTn9A). However, the present work shows that the transposon is situated within an unstable plasmid in this strain. A possibility of such plasmid's formation, as a result of a deletion of a part of bacteriophage lambda DNA, is shown.