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Abstract
Escherichia coli RecE protein is part of the classical RecET recombination system
that has recently been used in powerful new methods for genetic engineering. RecE
binds to free double-stranded DNA (dsDNA) ends and processively digests the 5'-ended
strand to form 5'-mononucleotides and a 3'-overhang that is a substrate for single
strand annealing promoted by RecT. Here, we report the crystal structure of the C-terminal
nuclease domain of RecE at 2.8 A resolution. RecE forms a toroidal tetramer with a
central tapered channel that is wide enough to bind dsDNA at one end, but is partially
plugged at the other end by the C-terminal segment of the protein. Four narrow tunnels,
one within each subunit of the tetramer, lead from the central channel to the four
active sites, which lie about 15 A from the channel. The structure, combined with
mutational studies, suggests a mechanism in which dsDNA enters through the open end
of the central channel, the 5'-ended strand passes through a tunnel to access one
of the four active sites, and the 3'-ended strand passes through the plugged end of
the channel at the back of the tetramer.