Activity and Effects of Retroelements in Bacteria*

Retroelements (RTEs) are abundant in eukaryotic genomes but less numerous in bacteria as group II introns. It has been hypothesized that eukaryotic spliceosomal introns and retrotransposons may have evolved as a result of invasion by bacterial group II introns. However, it remains unclear what limits RTE proliferation in bacteria and archaea and what enables it in eukaryotes. We quantify the effects of the human RTE LINE-1 and the bacterial group II intron Ll.LtrB in Escherichia coli and Bacillus subtilis. We find that RTE expression is detrimental to both species, that LINE-1 successfully integrates into the chromosomes, and that the ability to repair DNA breaks with bacterial non-homologous end joining systems increases retrotransposition efficiency. Our results show that RTEs place a significant burden on organisms poorly equipped to handle their effects, and that the capacity of the last eukaryotic common ancestor for NHEJ may have enabled the proliferation of RTEs and the evolution of eukaryotes.