Tn5 transposase loops DNA in the absence of Tn5 transposon end sequences

Authors

  • Christian D. Adams,

    1. Department of Biochemistry, University of Wisconsin at Madison, 433 Babcock Drive, WI 53706, USA.
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  • Bernhard Schnurr,

    1. Department of Physics, University of Illinois at Chicago, 845 West Taylor Street, Chicago, IL 60607, USA.
    2. Skirball Institute of Biomolecular Medicine and Department of Pathology, New York University School of Medicine, 540 First Avenue, New York, NY 10016, USA.
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  • Dunja Skoko,

    1. Department of Physics, University of Illinois at Chicago, 845 West Taylor Street, Chicago, IL 60607, USA.
    2. Laboratory of Molecular Biology, NIDDK, National Institutes of Health, Bethesda, MD 20892, USA. Departments of
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  • John F. Marko,

    1. Department of Physics, University of Illinois at Chicago, 845 West Taylor Street, Chicago, IL 60607, USA.
    2. Biochemistry, Molecular Biology and Cell Biology and
    3. Physics and Astronomy, Northwestern University, 2205 Tech Drive, Evanston, IL 60208, USA.
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  • William S. Reznikoff

    Corresponding author
    1. Department of Biochemistry, University of Wisconsin at Madison, 433 Babcock Drive, WI 53706, USA.
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*E-mail reznikoff@biochem.wisc.edu; Tel. (+1) 608 262 3608; Fax (+1) 608 265 2603.

Summary

Transposases mediate transposition first by binding specific DNA end sequences that define a transposable element and then by organizing protein and DNA into a highly structured and stable nucleoprotein ‘synaptic’ complex. Synaptic complex assembly is a central checkpoint in many transposition mechanisms. The Tn5 synaptic complex contains two Tn5 transposase subunits and two Tn5 transposon end sequences, exhibits extensive protein–end sequence DNA contacts and is the node of a DNA loop. Using single-molecule and bulk biochemical approaches, we found that Tn5 transposase assembles a stable nucleoprotein complex in the absence of Tn5 transposon end sequences. Surprisingly, this end sequence-independent complex has structural similarities to the synaptic complex. This complex is the node of a DNA loop; transposase dimerization and DNA specificity mutants affect its assembly; and it likely has the same number of proteins and DNA molecules as the synaptic complex. Furthermore, our results indicate that Tn5 transposase preferentially binds and loops a subset of non-Tn5 end sequences. Assembly of end sequence-independent nucleoprotein complexes likely plays a role in the in vivo downregulation of transposition and the cis-transposition bias of many bacterial transposases.

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