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Mol. Cell. Biol., Jan 1997, 296-308, Vol 17, No. 1
PW Hammond, TN Lively and TR Cech
Telomerase is a ribonucleoprotein enzyme that adds telomeric sequence
repeats to the ends of linear chromosomes. In vitro, telomerase has been
observed to add repeats to a DNA oligonucleotide primer in a processive
manner, leading to the postulation of a DNA anchor site separate from the
catalytic site of the enzyme. We have substituted photoreactive
5-iododeoxypyrimidines into the DNA oligonucleotide primer d(T4G4T4G4T4G2)
and, upon irradiation, obtained cross-links with the anchor site of
telomerase from Euplotes aediculatus nuclear extract. No cross-linking
occurred with a primer having the same 5' end and a nontelomeric 3' end.
These cross-links were shown to be between the DNA primer and (i) a protein
moiety of approximately 130 kDa and (ii) U51-U52 of the telomerase RNA. The
cross-linked primer could be extended by telomerase in the presence of
[alpha-32P]dGTP, thus indicating that the 3' end was bound in the enzyme
active site. The locations of the cross-links within the single-stranded
primers were 20 to 22 nucleotides upstream of the 3' end, providing a
measure of the length of DNA required to span the telomerase active and
anchor sites. When the single-stranded primers are aligned with the G-rich
strand of a Euplotes telomere, the cross-linked nucleotides correspond to
the duplex region. Consistent with this finding, a cross-link to telomerase
was obtained by substitution of 5-iododeoxycytidine into the CA strand of
the duplex region of telomere analogs. We conclude that the anchor site in
the approximately 130-kDa protein can bind duplex as well as
single-stranded DNA, which may be critical for its function at chromosome
ends. Quantitation of the processivity with single-stranded DNA primers and
double-stranded primers with 3' tails showed that only 60% of the primer
remains bound after each repeat addition.
Copyright © 1997, American Society for Microbiology
The anchor site of telomerase from Euplotes aediculatus revealed by photo-cross-linking to single- and double-stranded DNA primers
Howard Hughes Medical Institute, Department of Chemistry and Biochemistry, University of Colorado, Boulder 80309-0215, USA.
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