Known knowns, known unknowns and unknown unknowns in prokaryotic transposition

•Transposition of only a fraction of the extensive prokaryotic TE pool is understood.•Detailed transposition mechanisms of several classic and important TE remains unclear.•TE important in AbR spread (ISCR) and genome structure (REP) require a deeper understanding.•New potential TE e.g. CRISPR-related casposons and ISC are appearing from bioinformatics analyses.

Although the phenomenon of transposition has been known for over 60 years, its overarching importance in modifying and streamlining genomes took some time to recognize. In spite of a robust understanding of transposition of some TE, there remain a number of important TE groups with potential high genome impact and unknown transposition mechanisms and yet others, only recently identified by bioinformatics, yet to be formally confirmed as mobile. Here, we point to some areas of limited understanding concerning well established important TE groups with DDE Tpases, to address central gaps in our knowledge of characterised Tn with other types of Tpases and finally, to highlight new potentially mobile DNA species. It is not exhaustive. Examples have been chosen to provide encouragement in the continued exploration of the considerable prokaryotic mobilome especially in light of the current threat to public health posed by the spread of multiple AbR.