Active in vivo translocation of the Methanosarcina mazei Gö1 Casposon

Casposons are transposable elements containing the CRISPR associated gene Cas1solo. Identified in many archaeal genomes, casposons are discussed as the origin of CRISPR-Cas systems due to their proposed Cas1solo-dependent translocation. However, apart from bioinformatic approaches and the demonstration of Cas1solo integrase and endonuclease activity in vitro, casposon transposition has not yet been shown in vivo. Here, we report on active casposon translocations in Methanosarcina mazei Gö1 using two independent experimental approaches. First, mini-casposons, consisting of a R6Kγ origin and two antibiotic resistance cassettes, flanked by target site duplications (TSDs) and terminal inverted repeats (TIRs), were generated, and shown to actively translocate from a suicide plasmid and integrate into the chromosomal MetMaz-C1 TSD IS1a. Second, casposon excision activity was confirmed in a long-term evolution experiment using a Cas1solo overexpression strain in comparison to an empty vector control under four different treatments (native, high temperature, high salt, mitomycin C) to study stress-induced translocation. Analysis of genomic DNA using a nested qPCR approach provided clear evidence of casposon activity in single cells and revealed significantly different casposon excision frequencies between treatments and strains. Our results, providing the first experimental evidence for in vivo casposon activity are summarized in a modified hypothetical translocation model.