Targeted transposition with Tn7 elements: safe sites, mobile plasmids, CRISPR/Cas and beyond

Abstract

Summary Transposon Tn7 is notable for the control it exercises over where transposition events are directed. One Tn7 integration pathways recognizes a highly conserved attachment (att) site in the chromosome, while a second pathway specifically recognizes mobile plasmids that facilitate transfer of the element to new hosts. In this review, I discuss newly discovered families of Tn7‐like elements with different targeting pathways. Perhaps the most exciting examples are multiple instances where Tn7‐like elements have repurposed CRISPR/Cas systems. In these cases, the CRISPR/Cas systems have lost their canonical defensive function to destroy incoming mobile elements; instead, the systems have been naturally adapted to use guide RNAs to specifically direct transposition into these mobile elements. The new families of Tn7‐like elements also include a variety of novel att sites in bacterial chromosomes where genome islands can form. Interesting families have also been revealed where proteins described in the prototypic Tn7 element are fused or otherwise repurposed for the new dual activities. This expanded understanding of Tn7‐like elements broadens our view of how genetic systems are repurposed and provides potentially exciting new tools for genome modification and genomics. Future opportunities and challenges to understanding the impact of the new families of Tn7‐like elements are discussed.

Strategies for Developing CRISPR‐Based Gene Editing Methods in Bacteria

“…This feature is further exploited to create randomized transposon‐insertion mutant library in bacterial species. Intriguingly, Tn7, an extensively investigated transposon in E. coli , has the two transposition pathways . The site‐specific pathway utilizes a site‐specific DNA‐binding protein, TnsD (belongs to the TniQ family), which recognizes a short conserved sequence, attTn7 , present in many bacteria .…”

“…Intriguingly, Tn7, an extensively investigated transposon in E. coli , has the two transposition pathways . The site‐specific pathway utilizes a site‐specific DNA‐binding protein, TnsD (belongs to the TniQ family), which recognizes a short conserved sequence, attTn7 , present in many bacteria . In contrast, the randomized pathway requires TnsE, which searches for discontinuous DNA replication intermediates or 3′‐recessed DNA with the sliding clamp (DnaN) .…”

“…[204] The site-specific pathway utilizes a site-specific DNA-binding protein, TnsD (belongs to the TniQ family), which recognizes a short conserved sequence, attTn7, present in many bacteria. [204] In contrast, the randomized pathway requires TnsE, which searches for discontinuous DNA replication intermediates or 3′-recessed DNA with the sliding clamp (DnaN). [204] The randomized pathway is prone to target other mobile genetic elements, such as exogenous plasmids, maximizing its ability to horizontally transfer.…”

“…[204] In contrast, the randomized pathway requires TnsE, which searches for discontinuous DNA replication intermediates or 3′-recessed DNA with the sliding clamp (DnaN). [204] The randomized pathway is prone to target other mobile genetic elements, such as exogenous plasmids, maximizing its ability to horizontally transfer. Recent bioinformatics analyses uncovered that a set of Tn7-like transposons contain CRISPR-Cas systems.…”

Characterization of Tn7-like transposons and its related antibiotic resistance among Enterobacteriaceae from livestock and poultry in China

“…Transposition module encodes ve proteins required for two transposition path-ways, tnsA, tnsB, tnsC, tnsD, and tnsE (tnsABCDE) (12). The basic transposition machinery of Tn7 and how it directs transposition into the attTn7 site is well understood and has been the subject of multiple reviews (13,14). tnsABC are required for both Tn7 transposition pathways, but only allow transposition when they function with one of two target site selecting proteins, tnsD or tnsE (15,16).…”