A bacterial antiviral defense system, DRT10, employs a telomerase-like mechanism of RNA-templated tandem-repeat DNA synthesis through protein-primed reverse transcription directed by an A-B-A′ sequence pattern in its associated ncRNA, and phylogenetic analysis reveals that eukaryotic telomerase reverse transcriptase (TERT) is evolutionarily related to bacterial Class 2 UG reverse transcriptases, indicating that telomerase originated in bacteria before being co-opted for chromosome maintenance in eukaryotes.

Defense-associated reverse transcriptases (DRTs) employ diverse and distinctive mechanisms of cDNA synthesis to protect bacteria against viral infection. However, much of DRT family diversity remains unstudied. Here we identify a new antiviral defense system, DRT10, that associates with a non-coding RNA (ncRNA) to catalyze processive, protein-primed synthesis of tandem-repeat DNA. Repeat addition is dictated by sequence and structural features of the ncRNA that have direct parallels in the RNA component of telomerase. Remarkably, a phylogenetic analysis of RTs across domains of life reveals an unexpected link between DRT10 and telomerase that is further supported by structural comparisons and mechanistic evidence. These findings expand the repertoire of reverse transcription mechanisms in antiviral defense and point to a bacterial origin for telomerase.

Insights from antiviral defense systems reveal an unexpected bacterial origin for the mechanism of chromosome maintenance by eukaryotic telomerase.