Temporal self-regulation of transposition through host-independent transposase rodlet formation

Transposons are highly abundant in eukaryotic genomes, but their mobilization must be finely tuned to maintain host organism fitness and allow for transposon propagation. Forty percent of the human genome is comprised of transposable element sequences, and the most abundant cut-and-paste transposons are from the hAT superfamily. We found that the hAT transposase TcBuster from Tribolium castaneum formed filamentous structures, or rodlets, in human tissue culture cells, after gene transfer to adult mice, and ex vivo in cell-free conditions, indicating that host co-factors or cellular structures were not required for rodlet formation. Time-lapsed imaging of GFP-laced rodlets in human cells revealed that they formed quickly in a dynamic process involving fusion and fission. We delayed the availability of the transposon DNA and found that transposition declined after transposase concentrations became high enough for visible transposase rodlets to appear. In combination with earlier findings for maize Ac elements, these results give insight into transposase overproduction inhibition by demonstrating that the appearance of transposase protein structures and the end of active transposition are simultaneous, an effect with implications for genetic engineering and horizontal gene transfer.