Recruitment of CRISPR-Cas systems by Tn7-like transposons

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

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.

Publication types

  • Research Support, N.I.H., Intramural
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Bacteria / metabolism
  • CRISPR-Associated Proteins / metabolism
  • CRISPR-Cas Systems / genetics
  • CRISPR-Cas Systems / physiology*
  • Clustered Regularly Interspaced Short Palindromic Repeats
  • DNA Transposable Elements / genetics
  • DNA Transposable Elements / physiology*
  • Genes, Archaeal / genetics
  • Phylogeny
  • Plasmids
  • RNA, Guide
  • Sequence Analysis, RNA

Substances

  • CRISPR-Associated Proteins
  • DNA Transposable Elements
  • RNA, Guide