Phage-encoded TelN inhibits bacterial Mre11-Rad50 nuclease to protect hairpin telomeres

Ends of linear chromosomes require protection from host repair machinery that otherwise will mistake them for damaged DNA. The E. coli bacteriophage N15 harbors a linear genome with covalently closed hairpin ends formed by the phage-encoded telomere resolvase TelN. The double-strand break repair complex Mre11-Rad50 (MR, SbcCD in E. coli) specifically targets DNA termini, yet how hairpin telomeres evade host nuclease degradation in bacteria remains unknown. Here, we demonstrate that TelN is essential and sufficient to protect N15 phage-derived hairpin telomeres from MR processing in E. coli. Using a combination of genetic and biochemical approaches, we show that this protective function requires both TelN sequence-specific DNA binding and species-specific protein-protein interactions. Notably, we found that protection is independent of TelN's resolution activity and does not require the C-terminal domains of TelN. Our findings reveal a potentially broad mechanism of telomere protection, providing insights into a conserved regulation of MR activity at chromosome ends across the tree of life.