Formation of the tandem repeat (IS30)2 and its role in IS30-mediated transpositional DNA rearrangements (original) (raw)

Abstract

Plasmids carrying two IS_30_ elements in the same orientation, as in the composite transposon Tn_2706_, are structurally unstable in Escherichia coli. A primary segregation product is formed by site-specific deletion of the sequences carried between the two IS_30_ elements. The resulting covalently closed replicon carries the two IS_30_ elements as tandem repeats separated by only 2 bp. This (IS_30_)2 structure is extremely unstable, but it can nevertheless be isolated on its vector plasmid and, after purification, can be reintroduced into host cells by transformation. Among the descendants of transformants of recA − bacteria, replicated copies of the introduced (IS_30_)2 structure are still present, together with various kinds of segregation products which provide evidence for the efficient generation of DNA rearrangements. Most abundant is the product of another site-specific recombination between two identical ends of the IS_30_ elements involved, which results in the presence of just one intact IS_30_ on the plasmid. Apart from this, and depending on the presence of appropriate targets for IS_30_ transposition, various transposition products of (IS_30_)2 are also seen. Intramolecular reactions lead to DNA inversions and deletions with breakpoints other than IS_30_ ends. In intermolecular reactions inverse transposition occurs at high frequency and one also obtains simple transposition and cointegration. A mutational study revealed the requirement in cis of one intact IS_30_ transposase gene and of both proximal ends of the two IS_30_ elements concerned not only for the formation of (IS_30_)2, but also for its further rearrangement reactions, including the efficient formation of site-specific deletions. A model is proposed, which postulates that (IS_30_)2 intermediates play a key role in IS_30_ transposition pathways in which the formation of (IS_30_)2 may be rate-limiting. Once this structure is formed, it gives rise to a burst of transpositional rearrangements in the subclone carrying (IS_30_)2. Evolutionary implications of these findings are discussed.

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Authors and Affiliations

  1. Institute of Molecular Genetics, Agricultural Biotechnological Center, Szent-Györgyi A. u. 4, H-2101, Gödöllö, Hungary
    Ferenc Olasz
  2. Department of Microbiology, Biozentrum, University of Basel, Klingelbergstrasse 70, CH-4056, Basel, Switzerland
    Ferenc Olasz, Rolf Stalder & Werner Arber

Authors

  1. Ferenc Olasz
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  2. Rolf Stalder
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  3. Werner Arber
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Communicated by H. Hennecke

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Olasz, F., Stalder, R. & Arber, W. Formation of the tandem repeat (IS_30_)2 and its role in IS_30_-mediated transpositional DNA rearrangements.Molec. Gen. Genet. 239, 177–187 (1993). https://doi.org/10.1007/BF00281616

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