In pioneering studies performed, Carol W. Greider and Elizabeth Blackburn isolated the 159-nucleotide RNA subunit of Tetrabymena telomerase and discovered that the RNA contains an internal 5'-CAACCCCAA-3'sequence, which is complementary to the d (TTGGGG)n telomeric repeat in Tetrabymena. Subsequent studies revealed that telomerase RNA molecules from other organisms differ in size, sequence, or both. Nevertheless, each telomerase RNA molecule has a sequence that is complementary to the telomeric DNA of the organism from which the telomerase was isolated. These resuits suggest that a short sequence within telomerase RNA acts as a template for telomeric DNA synthesis.
Further proof for this hypothesis comes from experiments that show telomerase activity is lost after RNase digestion but can be restored by removing RNase and adding back intact RNA. When the added RNA has a nucleotide substitution in its template sequence, the telomeric DNA produced has the predicted change. Genetic studies with S.cerevisiae also indicate that telomerase RNA is essential for telomerase function because the gene that specifies telomerase RNA (TLC1) is essential for telomere maintenance. Thus, telomerase acts as an RNA-dependent DNA polymerase or reverse transcriptase that supplies its own template RNA molecule.