[Transposition of the composite synthetic transposon TnV (Tn5-Rep(pSC101)) is accompanied by the formation of the mini-plasmid pTnV, containing defective Is50-elements].

Using thermoelimination (at 42 degrees C) of the thermoinducible coliphage P1tsCmr omega::TnV (TnV is a Tn5 derivative which contains the replication origin (Rep) of plasmid pSC101), more than 110 KmrCms Escherichia coli K12 clones were selected. It was supposed that the KmrCms phenotype could result only from insertion of TnV (Kmr) into E. coli chromosome and the loss of phage (Cms). It was found that the majority of KmrCms clones (35-90%) contained miniplasmids. Their molecular sizes did not exceed the TnV size (6.1 kb). The formation of miniplasmids called pTnV was observed both in RecA+ cells (C600) and in RecA- (HB101), more often in the latters. Interestingly, that miniplasmids of only several molecular sizes were detected: from 6.1 kb (pTnV60) to 4.35 kb (pTnV43). A restriction analysis showed that DNA of the majority of pTnV plasmids had varying deletions (0.3-1.3 kb) of mainly IS50L element which together with IS50R flank TnV. Very low transposition frequencies (approx. 10(-8) Kmr transconjugants per transferred R388) of all pTnV types (including pTnV60 plasmids containing probably microdeletions of the joining "outside" IS50's ends) suggest that pTnV plasmids are not intermediates in TnV transposition. Possibly the circularized TnV derivatives (pTnV's) are side products of the transposition resulting from the abortive attempts of an excised and autonomous transposon molecule to insert into itself. In the present paper the possible mechanisms of the origin of limited pTnV type numbers are also discussed.