The molecular mechanism for TERRA recruitment and annealing to telomere.

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Telomeres are DNA-protein complexes that protect the ends of linear chromosomes. Telomeres are composed of tandem repeat sequences of duplexed (TTAGGG)n, ending with a 3’ single stranded (TTAGGG)n sequence bound to telomere specific proteins known as shelterin complex. In healthy somatic cells, progressive loss of telomeres results in cellular senescence. In 85% of cancer cells, however telomeres are elongated by telomerase, while the remaining 15% of cancers use alternative lengthening of telomeres (ALT). In ALT (+) cancers, Telomeric repeat containing RNA (TERRA), a noncoding RNA that is transcribed from telomeres is overexpressed. TERRA trans anneals by invading the telomeric duplex to form an R-loop. Recent study showed that R-loop formation is promoted by Rad51 and antagonized by RNaseH1. Despite the evidence of TERRA's role in telomere structure and function, our molecular understanding of TERRA is limited. Here, we elucidate the molecular mechanism underlying TERRA recruitment and invasion into telomeres in the context of shelterin, RAD51 and RNaseH1 using single molecule (sm) assays. Using smFRET we show that TERRA trans anneals to telomeric DNA and exhibits dynamic movement that is stabilized by TRF2. Additionally, TERRA annealing to the telomeric duplex forms a triplex structure. Sm-colocalization assays show that the presence of non-TERRA sequence, which represents the sub-telomeric DNA and the presence of G-quadruplex telomeric overhang enhances TERRA binding to telomeric duplex. We also studied the mechanism underlying TERRA recruitment and invasion by Rad51 and shelterin proteins. Our result shows that RAD51-TERRA complex invades telomere duplex more efficiently than TERRA alone. Additionally, TRF2 increases TERRA affinity to telomeric duplex and protects it from RNaseH1 digestion. In contrast, TRF1 represses TERRA binding to telomeric duplex and does not provide protection against RNaseH1. We demonstrate the molecular mechanism underpinning TERRA recruitment and annealing to telomere.