| Summary: | Summary: Biogenesis of the human telomerase RNA (hTR) involves a complex series of posttranscriptional modifications, including hypermethylation of the 5′ mono-methylguanosine cap to a tri-methylguanosine cap (TMG). How the TMG cap affects hTR maturation is unknown. Here, we show that depletion of trimethylguanosine synthase 1 (TGS1), the enzyme responsible for cap hypermethylation, increases levels of hTR and telomerase. Diminished trimethylation increases hTR association with the cap-binding complex (CBC) and with Sm chaperone proteins. Loss of TGS1 causes an increase in accumulation of mature hTR in both the nucleus and the cytoplasm compared with controls. In TGS1 mutant cells, increased hTR assembles with telomerase reverse transcriptase (TERT) protein to yield elevated active telomerase complexes and increased telomerase activity, resulting in telomere elongation in cultured human cells. Our results show that TGS1-mediated hypermethylation of the hTR cap inhibits hTR accumulation, restrains levels of assembled telomerase, and limits telomere elongation. : hTR, the RNA component of telomerase, acquires a trimethylguanosine cap synthesized by Trimethylguanosine synthase 1 (TGS1). Chen et al. show that TGS1 and cap hypermethylation control hTR abundance and intracellular distribution. Loss of TGS1 results in elevated hTR levels, increased telomerase activity and telomere elongation. Keywords: TGS1, hTR, TERT, telomerase, telomeres, cap hypermethylation, telomere elongation
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