RAD51-dependent recruitment of TERRA lncRNA to telomeres through R-loops

Telomeres which are present at the ends of eukaryotic chromosomes mediate genome stability and determine cellular lifespan(1). Telomeric repeat containing RNAs (TERRA) are long noncoding RNAs transcribed from chromosome ends(2,3), which regulate telomeric chromatin structure and telomere maintenance via telomerase and homology directed repair (HDR)(4,5). The mechanisms by which TERRA is recruited to chromosome ends remain poorly defined. Here we develop a reporter system to dissect the underlying mechanisms and demonstrate that the UUAGGG-repeats of TERRA are both necessary and sufficient to target TERRA to chromosome ends. TERRA preferentially associates with short telomeres through the formation of telomeric DNA:RNA hybrid (R-loop) structures that can form in trans. Telomere association and R-loop formation triggers telomere fragility and is promoted by the RAD51 recombinase and its interacting partner BRCA2 but counteracted by RNA surveillance factors, RNaseH1 and TRF1. RAD51 physically interacts with TERRA and catalyzes R-loop formation with TERRA in vitro supporting a direct involvement of this DNA recombinase in TERRA recruitment by strand invasion. Together, our findings reveal a RAD51-dependent pathway that governs TERRA mediated R-loop formation post transcription providing a mechanism of how lncRNAs can be recruited to new loci in trans.