Sequence-specific processing of telomeric 3′ overhangs by the Werner syndrome protein exonuclease activity

Werner syndrome is a premature aging disease caused by loss of function mutations in the Werner syndrome(WRN) gene. WRN is a RecQ helicase that in contrast to every other member of this family of proteins possesses an exoactivity. The findings that cells lacking WRN activity display accelerated telomere shortening and WRN can be detchromosome ends suggest that this protein participates in some aspects of telomere metabolism. In this study we examimpact of WRN on telomeric substrates with a 3' single-stranded overhang in vitro and show that WRN has sequence-exonuclease activity that removes several nucleotides inward with a periodical pattern from the 3' end of the telomeric ov This activity is strictly dependent on the presence of telomeric sequences in both the duplex DNA and 3' overhang DNA and is strongly inhibited by the telomeric factor POT1 but not TRF2. These data demonstrate that WRN processes telome substrates with a 3' single-stranded overhang with high specificity and suggest that this protein could influence the configutelomere ends prior to the formation of a protective t-loop structure.