Importance of illegitimate recombination and transposition in IS30-associated excision events

In the present study we report on the excision of IS30 elements and IS30-de rived composite transposons. Frequent loss of IS30 was observed during diss olution of dimeric IS30 structures, containing LR-IR junctions, leading to resealed donor molecules. In contrast, unambiguous transpositional excision resulting in resealed remainder products could not be identified in the ca se of a monomeric element. The bias in the excision of monomeric and dimeri c IS30 structures indicates a difference in the molecular mechanism of tran sposition of IS30 monomers and dimers. Sequence data on the rarely detected plasmids missing full IS or Tn copies rather suggest that all products wer e derived from illegitimate recombination. The reaction occurred between sh ort homologies and was independent of the transposase activity. Similar IS3 0 excision events accompanied by multiple plasmid or genome rearrangements were detected in Pseudomonas putida and Rhizobium meliloti, yielding stable replicons that retained the selective marker gene of the transposon. We pr ovide evidence that both transposition and illegitimate recombination can c ontribute to the stabilization of replicons through the elimination of IS e lements, which emphasizes the evolutionary significance of these events. (C ) 1999 Academic Press.