Werner syndrome (WS) is a premature aging syndrome caused by mutations in the WS gene (WRN) and a deficiency in the function of the Werner protein (WRN). WRN is a multifunctional nuclear protein that catalyzes three DNA-dependent reactions: a 3-5-exonuclease, an ATPase, and a 3-5-helicase. Deficiency in WRN results in a cellular phenotype of genomic instability. The biochemical characteristics of WRN and the cellular phenotype of WRN mutants suggest that WRN plays an important role in DNA

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... lic pathways such as recombination, transcription, replication, and repair. The catalytic activities of WRN have been extensively studied and are fairly well understood. However, much less is known about the domain-specific interactions between WRN and its DNA substrates. This study identifies and characterizes three distinct WRN DNA binding domains using recombinant truncated fragments of WRN and five DNA substrates (long forked duplex, blunt-ended duplex, single-stranded DNA, 5-overhang duplex, and Holliday junction). Substrate-specific DNA binding activity was detected in three domains, one Nterminal and two different C-terminal WRN fragments (RecQ conserved domain and helicase RNase D conserved domain-containing domains). The substrate specificity of each DNA binding domain may indicate that each protein domain has a distinct biological function. The importance of these results is discussed with respect to proposed roles for WRN in distinct DNA metabolic pathways. Werner syndrome (WS) 1 is a rare autosomal recessive disorder characterized by the early onset of aging symptoms, such as graying of the hair, cataracts, osteoporosis, atherosclerosis, type II diabetes mellitus, and high incidence of malignant neoplasm (1). The gene mutated in WS (WRN), encodes a nuclear 1432-amino acid protein (WRN) (2). WRN belongs to the