Telomerase is a reverse transcriptase that maintains chromosome ends. The N-terminal half of the catalytic protein subunit (TERT) contains three functional domains (I, II, and III) that are conserved among TERTs but not found in other reverse transcriptases. Guided by an amino acid sequence alignment of nine TERT proteins, mutations were introduced into yeast TERT (Est2p). In support of the proposed alignment, mutation of virtually all conserved residues resulted in loss-of-function or temperature sensitivity, accompanied by telomere shortening. Overexpression of telomerase component Est3p led to allele-specific suppression of the temperature-sensitive mutations in region I, suggesting that Est3p interacts with this protein domain. As predicted by the genetic results, a lethal mutation in region I resulted in loss of Est3p from the telomerase complex. We conclude that Est2p region I is required for the recruitment of Est3p to yeast telomerase. Given the phylogenetic conservation of region I of TERT, this protein domain may provide the equivalent function in all telomerases.