Exonuclease I (ExoI) from Escherichia coli is a monomeric enzyme that processively degrades single stranded DNA in the 3' to 5' direction and has been implicated in DNA recombination and repair. Determination of the structure of ExoI to 2.4 A resolution using X-ray crystallography verifies the expected correspondence between a region of ExoI and the exonuclease (or proofreading) domains of the DNA polymerases. The overall fold of ExoI also includes two other regions, one of which extends the exonuclease domain and another that can be described as an elaborated SH3 domain. These three regions combine to form a molecule that is shaped like the letter C, although it also contains a segment that effectively converts the C into an O-like shape. The structure of ExoI thus provides additional support for the idea that DNA metabolizing enzymes achieve processivity by completely enclosing the DNA.
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