IS200/IS605 Family-Associated TnpB Increases Transposon Activity and Retention

The IS200/IS605 family of insertion sequences are abundant mobile elements associated with one of the most numerous genes found in nature, tnpB1-3. Previous studies suggest that TnpB protein may be an evolutionary precursor to CRISPR Cas enzymes, and TnpB has received renewed interest having itself been shown to function as a Cas-like RNA-guided DNA endonuclease3,4. However, interpretation of the fundamental role of TnpB in transposition and how it contributes to genome dynamics5 remains controversial without direct, real-time measurement in live cells. Here, using a suite of fluorescent reporters coupled to transposition in live Escherichia coli, we show that IS608-TnpB causes increased transposon activity, and assists in preventing transposon loss from host genomes. Analyzing our results through a mathematical model of transposon dynamics, we discuss the multifaceted roles it may play in transposon regulation. The mutually beneficial transposon-TnpB interaction may explain the prevalence of tnpB, creating conditions for the appropriation of TnpBs RNA-guided endonuclease activity for adaptive immunity. SIGNIFICANCE STATEMENTPhylogenetic evidence suggests that tnpB, one of the most numerous genes found in nature, is the ancestral form of CRISPR-Cas enzymes and played a critical role in the evolution of adaptive immunity. However, the role TnpB plays in transposition that has contributed to its wide distribution remains unclear. Here, we use a unique approach that couples fluorescent reporters to transposition to non-perturbatively quantify transpositional dynamics in live cells. In contrast to previous indirect methods suggesting that TnpB suppresses transposition, our results instead clearly demonstrate that TnpB significantly increases transposition rates and enhances transposon retention within the host genome, resulting in a mutually beneficial interaction between transposons and TnpB that can account for its wide distribution.