Telomerase reverse transcriptase thumb DNA binding domainThe catalytic subunit of telomerase is structurally similar to retroviral reverse transcriptases, viral RNA polymerases and, to a lesser extent, the bacteriophage B-family DNA polymerases. Like its structural homologs, the core catalytic subunit of telomerase, TERT, contains the fingers, palm and thumb domains required for nucleic acid and nucleotide associations as well as catalysis. The four major TERT domains: the RNA binding domain (TRBD); the fingers domain, implicated in nucleotide binding and processivity; the palm domain, which contains the active site of the enzyme; and the thumb domain, implicated in DNA binding and processivity are organized into a ring configuration similar to that observed for the substrate-free enzyme. This is the thumb domain found in Tribolium castaneum telomerase catalytic subunit, TERT. Contacts between TERT and the DNA substrate are mostly mediated via backbone interactions with the thumb loop and helix. The thumb helix sits in the minor groove of the RNA-DNA heteroduplex, making extensive contacts with the phosphodiester backbone and the ribose groups of the RNA-DNA hybrid.