Structural basis for the evolution of a domesticated group II intron-like reverse transcriptase to function in host cell DNA repair

A previous study found that a domesticated bacterial group II intron-like reverse transcriptase (G2L4 RT) functions in double-strand break repair (DSBR) via microhomology-mediated end joining (MMEJ) and that a mobile group II intron-encoded RT has a basal DSBR activity that uses conserved structural features of non-LTR-retroelement RTs. Here, we determined G2L4 RT apoenzyme and snap-back DNA synthesis structures revealing novel structural adaptations that optimized its cellular function in DSBR. These included a unique RT3a structure that stabilizes the apoenzyme in an inactive conformation until encountering an appropriate substrate; a longer N-terminal extension/RT0-loop with conserved residues that together with a modified active site favors strand annealing; and a conserved dimer interface that localizes G2L4 RT homodimers to DSBR sites with both monomers positioned for MMEJ. Our findings reveal how a non-LTR-retroelement RT evolved a dedicated cellular function and suggest new ways of optimizing these RTs for genome engineering applications.