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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.

Author information

1
Department of Microbiology, Cornell University, Ithaca, NY 14853; joe.peters@cornell.edu koonin@ncbi.nlm.nih.gov.
2
National Center for Biotechnology Information, National Institutes of Health, Bethesda, MD 20894.
3
Skolkovo Institute of Science and Technology, Skolkovo, 143025, Russia.
4
National Center for Biotechnology Information, National Institutes of Health, Bethesda, MD 20894; joe.peters@cornell.edu koonin@ncbi.nlm.nih.gov.

Abstract

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.

KEYWORDS:

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

PMID:
28811374
PMCID:
PMC5584455
DOI:
10.1073/pnas.1709035114
[Indexed for MEDLINE]
Free PMC Article

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