Host exonuclease SbcB and a phage-encoded SSB-like protein control activation of the DRT10 reverse transcriptase defense system

Defense-associated reverse transcriptases (DRTs) employ diverse mechanisms of cDNA synthesis to protect bacteria against phage infection, yet their full diversity and regulatory logic remain poorly understood. Here we provide a mechanistic characterization of UG17 (DRT10), a class-2 system within the UG/Abi reverse transcriptase lineage, classifying it into three phylogenetically and architecturally distinct subtypes with subtype-specific ncRNAs and experimentally validating its role in phage defense. DRT10 operates as a tripartite module, comprising a structured ncRNA, a SLATT effector, and an RT that catalyzes processive synthesis of DNA containing 7 nt tandem repeats. The tandem-repeat cDNA intermediate accumulates as both first- and second-strand species. SbcB suppresses DRT10-mediated toxicity and appears to selectively degrade the second strand under basal conditions, while phage infection correlates with enhanced first-strand accumulation. A phage-encoded protein with predicted SSB-like architecture and a conserved C-terminal tip motif is required for efficient DRT10 activation during infection. Together, these findings establish DRT10 as a surveillance system whose activation threshold is jointly controlled by constitutive cDNA synthesis, host exonuclease activity, and a phage-encoded SSB-like trigger that perturbs host ssDNA metabolism. We propose a model in which accumulation of DRT10-derived cDNA triggers activation of the SLATT transmembrane effector to initiate immune defense.