M. Melek et al., PROCESSING OF NONTELOMERIC 3' ENDS BY TELOMERASE - DEFAULT TEMPLATE ALIGNMENT AND ENDONUCLEOLYTIC CLEAVAGE, Molecular and cellular biology, 16(7), 1996, pp. 3437-3445
Telomerase is a specialized reverse transcriptase that maintains telom
eres at chromosome ends by extending preexisting tracts of telomeric D
NA and forming telomeres de novo an broken chromosomes. Whereas the in
teraction of telomerase with telomeric DNA has been studied in some de
tail, relatively little is known about how this enzyme processes nonte
lomeric DNA. In this study we recruited the Euplotes telomerase to non
telomeric 3' termini in vitro using chimeric DNA primers that carried
one repeat of a telomeric sequence at various positions upstream of a
nontelomeric 3' end. Such primers were processed in two distinct pathw
ays. First, nontelomeric 3' ends could be elongated directly by positi
oning a primer terminus at a specific site on the RNA template. Delive
ry to this default site was precise, always resulting in the addition
of 4 dG residues' to the nontelomeric 3' ends. These same residues ini
tiate new telomeres formed in vivo. Alternatively, 3' nontelomeric nuc
leotides were removed from primers prior to initiating the first elong
ation cycle. As with default positioning df nontelomeric 3' ends, the
cleavage event was' extremely precise and was followed by the addition
of dG residues to the primer 3' ends. The specificity of the cleavage
reaction was mediated by primer interaction with the RNA template and
, remarkably, proceeded by an endonucleolytic mechanism. These observa
tions suggest a mechanism for the precision of developmentally regulat
ed de nova telomere formation and expand our understanding bf the enzy
matic properties of telomerase.