Structure of the Tetrahymena thermophila telomerase RNA helix II template boundary element

Telomere addition by telomerase requires an internal templating sequence located in the RNA subunit of telomerase. The correct boundary definition of this template sequence is essential for the proper addition of the nucleotide repeats. Incorporation of incorrect telomeric repeats onto the ends of chromosomes has been shown to induce chromosomal instability in ciliate, yeast and human cells. A 5' template boundary defining element (TBE) has been identified in human, yeast and ciliate telomerase RNAs. Here, we report the solution structure of the TBE element (helix II) from Tetrahymena thermophila telomerase RNA. Our results indicate that helix II and its capping pentaloop form a well-defined structure including unpaired, stacked adenine nucleotides in the stem and an unusual syn adenine nucleotide in the loop. A comparison of the T.thermophila helix II pentaloop with a pentaloop of the same sequence found in the 23S rRNA of the Haloarcula marismortui ribosome suggests possible RNA and/or protein interactions for the helix II loop within the Tetrahymena telomerase holoenzyme.