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Proc Natl Acad Sci U S A. 2017 Aug 29;114(35):E7358-E7366. doi: 10.1073/pnas.1709035114. Epub 2017 Aug 15.

Recruitment of CRISPR-Cas systems by Tn7-like transposons.

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Department of Microbiology, Cornell University, Ithaca, NY 14853;
National Center for Biotechnology Information, National Institutes of Health, Bethesda, MD 20894.
Skolkovo Institute of Science and Technology, Skolkovo, 143025, Russia.
National Center for Biotechnology Information, National Institutes of Health, Bethesda, MD 20894;


A survey of bacterial and archaeal genomes shows that many Tn7-like transposons contain minimal type I-F CRISPR-Cas systems that consist of fused cas8f and cas5f, cas7f, and cas6f genes and a short CRISPR array. Several small groups of Tn7-like transposons encompass similarly truncated type I-B CRISPR-Cas. This minimal gene complement of the transposon-associated CRISPR-Cas systems implies that they are competent for pre-CRISPR RNA (precrRNA) processing yielding mature crRNAs and target binding but not target cleavage that is required for interference. Phylogenetic analysis demonstrates that evolution of the CRISPR-Cas-containing transposons included a single, ancestral capture of a type I-F locus and two independent instances of type I-B loci capture. We show that the transposon-associated CRISPR arrays contain spacers homologous to plasmid and temperate phage sequences and, in some cases, chromosomal sequences adjacent to the transposon. We hypothesize that the transposon-encoded CRISPR-Cas systems generate displacement (R-loops) in the cognate DNA sites, targeting the transposon to these sites and thus facilitating their spread via plasmids and phages. These findings suggest the existence of RNA-guided transposition and fit the guns-for-hire concept whereby mobile genetic elements capture host defense systems and repurpose them for different stages in the life cycle of the element.


CRISPR-Cas systems; Tn7 transposon; crRNA guide; target-site selection; transposition strategy

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