Regulation of 5' template usage and incorporation of noncognate nucleotides by human telomerase.

Telomerase accurately synthesizes telomeric DNA by reverse transcription of a tightly defined template region in the telomerase RNA (TR). Reverse transcription past the 5' boundary of the template can cause the incorporation of noncognate nucleotides into telomeric DNA, which can result in disruption of normal telomere function. The products synthesized by human telomerase do not contain the nucleotide cytosine, which is encoded by an hTR residue 2 nucleotides (nt) 5' of the template boundary. We examined dCTP incorporation by a series of telomerases reconstituted with N- and C-terminally mutated human telomerase reverse transcriptases (hTERTs). We found that altering sequences in the N-terminal RNA interaction domain 1 (RID1) and C terminus caused dCTP-dependent catalytic phenotypes suggestive of reverse transcription of sequences 5' of the template boundary. A RID1 mutant that exhibited a dCTP-dependent phenotype interacted less efficiently with a human telomerase RNA (hTR) variant in which the 5' template boundary-defining P1b element was disrupted, whereas C-terminal mutations did not alter hTR interactions in a P1b-dependent fashion. Disruption of P1b or template linker sequences between P1b and the 5' template boundary also impaired 5' template usage in RID1 and C-terminal hTERT mutants. These observations identify overlapping roles for hTR sequences and structures 5' of the template in regulating both 5' template boundary definition and 5' template usage, and implicate hTERT N- and C-terminal regions in 5' template usage and suppression of noncognate nucleotide incorporation.