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The protein RAP1 is essential for the maintenance of the telomeres of
Saccharomyces cerevisiae and binds in vitro to multiple sites found within the
TG1-3 telomeric repeats. We show here that, in addition to its known binding
activity for double-stranded DNA, RAP1 binds sequence-specifically to the
GT-strands. This indicates that RAP1 is the protein that binds to the telomeric
terminal GT-tails. Furthermore, we have found that RAP1 binds to and promotes
the formation of G-tetrads, i.e. DNA quadruplexes, in GT-strand oligonucleotides
at nanomolar concentrations. The formation of DNA quadruplexes appears to
involve the intermolecular association of GT-strands. The minimal DNA-binding
domain of RAP1 (DBD) binds only to double-stranded DNA, so that the novel
DNA-binding activity we have found involves regions of the protein located
outside of the DBD. The finding that a telomeric protein promotes the formation
of G-tetrads argues for the use of DNA quadruplexes in telomere association.
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