Mobile genetic elements drive genome evolution and horizontal gene transfer, yet the mechanisms underlying the mobility of many eukaryotic elements remain poorly understood. Here we show that Starships, a recently discovered class of giant transposable elements in fungi, mobilise via an extrachromosomal circular intermediate. Using deep sequencing, we detect low-abundance reads spanning the hypothesised junction consistent with circularisation. We demonstrate that a synthetic circular intermediate, or "starmid", is sufficient for recombinase-dependent integration into the host genome and that circularisation enhances expression of the starship-encoded tyrosine recombinase through juxtaposition of 3' and 5' terminal sequences. This topology-dependent regulation parallels bacterial integrative elements, in which excision activates recombination machinery. Exploiting this circular intermediate, we explore starmids as a platform for transient DNA delivery, enabling CRISPR Cas9-mediated genome editing in Paecilomyces variotii with subsequent curing of the construct. Together, our results shed light on the biology of starship mobility and suggest promising biotechnology applications for these elements.
Baericke, A., Jameson, V., Urquhart, A. S.
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