DNA replication machinery copies an entire genome with remarkable speed and accuracy in preparation for each cell division. However, DNA polymerase unidirectionality and requirement for a free 3' hydroxyl group render these enzymes incapable of duplicating the extreme ends of eukaryotic linear chromosomes. This deficiency leads to a gradual loss of terminal sequence with each round of replication that threatens the integrity of essential genetic information. This calamity is avoided through the function of telomeres, typically structures of short repeating sequence added to chromosome 3' ends by telomerase (1). Soon after its discovery as a polymerase, telomerase was found to reverse transcribe a template sequence contained within the telomerase integral RNA subunit TER (2). Telomerase biochemical activity has proven to be unique even among reverse transcriptases (RTs) in features including the release of single-stranded rather than duplex product DNA, which is necessary for regenerating the template for telomerase's distinctive repeat synthesis processivity (3). How telomerase gained new types of nucleic acid handling specificity relative to other RTs has been the subject of intense research, much of which has focused on the structure and function of nontemplate specializations of TER and unique domains of the telomerase protein RT subunit TERT (4). In PNAS, Brown et al. present surprising evidence of a previously missed specialization of human telomerase (5). Using elegant and incisive biochemical assays, the authors demonstrate that this enzyme reads sequence cues within the product-template duplex to dramatically influence the specificity of product synthesis. The work reveals an unanticipated divergence of telomerase from other polymerases in active site adaptation for its cellular duty.
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