This domain is responsible for the 3'-5' exonuclease proofreading activity of E. coli DNA polymerase I (polI) and other enzymes, it catalyses the hydrolysis of unpaired or mismatched nucleotides. This domain consists of the amino-terminal half of the Klenow fragment in E. coli polI it is also found in the Werner syndrome helicase (WRN), focus forming activity 1 protein (FFA-1) and ribonuclease D (RNase D). Werner syndrome is a human genetic disorder causing premature aging; the WRN protein has helicase activity in the 3'-5' direction [4,5]. The FFA-1 protein is required for formation of a replication foci and also has helicase activity; it is a homologue of the WRN protein [3]. RNase D is a 3'-5' exonuclease involved in tRNA processing. Also found in this family is the autoantigen PM/Scl thought to be involved in polymyositis-scleroderma overlap syndrome.

1: Structure of large fragment of Escherichia coli DNA polymerase I complexed with dTMP. Ollis DL, Brick P, Hamlin R, Xuong NG, Steitz TA; Nature 1985;313:762-766. PMID:3883192

2: The proofreading domain of Escherichia coli DNA polymerase I and other DNA and/or RNA exonuclease domains. Moser MJ, Holley WR, Chatterjee A, Mian IS; Nucleic Acids Res 1997;25:5110-5118. PMID:9396823

3: Replication focus-forming activity 1 and the Werner syndrome gene product Yan H, Chen CY, Kobayashi R, Newport J; Nat Genet 1998;19:375-378. PMID:9697700

4: The Werner syndrome protein is a DNA helicase. Gray MD, Shen JC, Kamath-Loeb AS, Blank A, Sopher BL, Martin GM, Oshima J, Loeb LA; Nat Genet 1997;17:100-103. PMID:9288107

5: DNA helicase activity in Werner's syndrome gene product synthesized in a baculovirus system. Suzuki N, Shimamoto A, Imamura O, Kuromitsu J, Kitao S, Goto M, Furuichi Y; Nucleic Acids Res 1997;25:2973-2978. PMID:9224595