Reevaluation of the RNA binding properties of the Tetrahymena thermophila telomerase reverse transcriptase N-terminal domain

Telomerase restores chromosome-capping telomeric repeats lost with each round of genome replication by DNA-templated DNA polymerases. The telomerase reverse transcriptase (TERT) N-terminal (TEN) domain is a peripheral, telomerase-specific, processivity-stimulatory addition to more conserved domains that encircle the active site cavity. Reports of ciliate, yeast, and mammalian telomerase TEN domain associations with the telomerase RNA subunit (TR) describe low affinity interactions of uncertain specificity. Unfortunately two cryo-EM structures of synthesis-paused telomerase holoenzymes lack sufficient resolution to discriminate molecular specificity of possible TR contact(s) with the TEN domain, and there is no assigned density for the TEN domain termini implicated in RNA binding. Furthermore, studies have revealed alternative secondary structures for TR regions that could interact with TERT prior to TR folding into active conformation. Informed by recent advances in knowledge of telomerase structure, we returned to the investigation of Tetrahymena thermophila TERT TEN domain interaction with TR. Instead of finding specificity for a particular TR sequence or structure, we discovered that the tagged TEN domain used in previous characterizations has trace contamination with a bacterial RNA-interacting protein not detectable by SDS-PAGE. By resolving this interference, we show that the TEN domain binds RNAs dependent on RNA length rather than sequence.