Functional RNA splitting drove the evolutionary emergence of type V CRISPR-Cas systems from transposons.

Abstract

Transposon-encoded TnpB nucleases gave rise to type V CRISPR-Cas12 effectors through multiple independent domestication events. These systems use different RNA molecules as guides for DNA targeting: transposon-derived right-end RNAs (reRNAs or omega RNAs) for TnpB and CRISPR RNAs for type V CRISPR-Cas systems. However, the molecular mechanisms bridging transposon activity and CRISPR immunity remain unclear. We identify TranCs (transposon-CRISPR intermediates) derived from distinct IS605- or IS607-TnpB lineages. TranCs utilize both CRISPR RNAs and reRNAs to direct DNA cleavage. The cryoelectron microscopy (cryo-EM) structure of LaTranC from Lawsonibacter sp. closely resembles that of the ISDra2 TnpB complex; however, unlike a single-molecule reRNA, the LaTranC guide RNA is functionally split into a tracrRNA and crRNA. An engineered RNA split of ISDra2 TnpB enabled activity with a CRISPR array. These findings indicate that functional RNA splitting was the primary molecular event driving the emergence of diverse type V CRISPR-Cas systems from transposons.