J. Bednenko et al., REITERATIVE DG ADDITION BY EUPLOTES-CRASSUS TELOMERASE DURING EXTENSION OF NONTELOMERIC DNA, Nucleic acids research, 26(17), 1998, pp. 3998-4004
Telomerase from the ciliate Euplotes crassus incorporates G(4)T(4) tel
omeric repeats onto both telomeric and non-telomeric single-stranded D
NA 3'-ends via reverse transcription of a templating domain in its RNA
subunit. Here we describe an unusual mode of template copying that is
characteristic of DNA synthesis onto nontelomeric 3'-ends in vitro. W
hen dTTP was eliminated from telomerase reactions, telomeric primers o
r DNA products generated from the telomerase endonuclease were extende
d by precise copying of the RNA template. In contrast, telomerase cata
lyzed the addition of up to 13 dG residues onto primers with non-telom
eric 3'-ends under the same reaction conditions. Introducing mismatche
s in the 3'-terminus of telomeric primers that reduced primer compleme
ntarity to the RNA template induced reiterative dG incorporation, indi
cating that the reaction is influenced by Watson-Crick base pair forma
tion between the primer and the RNA template. Unexpectedly, the reiter
ative dG addition mode was confined to telomerase derived from develop
ing cells that undergo new telomere formation. This reaction was not o
bserved in vegetatively growing cells. We postulate that indiscriminat
e dG addition by telomerase occurs by reiterative copying of C residue
s in the telomerase RNA templating domain and reflects lateral instabi
lity of the primer-template interaction during de novo telomere format
ion.