Fast-scan AFM observations of restriction enzyme

The excision and repair of mismatched base pairs is an essential task of cell maintenance. Several models have been proposed to describe how the hemimethylated d(GATC) sequence can direct Escherichia coli mismatch repair while residing on either side of a mismatch, at a separation distance of 1,000 bp or more. Anna Pluciennik and Paul Modrich report that a protein roadblock and breaks in the DNA double helix can disrupt the repair process, which could eliminate some models from consideration. The authors placed a hydrolytically defective form of EcoRI endonuclease on the helix between a d(GATC) sequence and a mismatch to attempt to block signaling along the helix. The blocking protein inhibited MutH endonuclease activation up to 80% of the time. Using a circular DNA strand containing a mismatch and one d(GATC) site, they also showed that a break in the double helix located within the shorter path between the two sites blocked MutH activation, but a break placed within the longer path did not. The authors suggest that signals are transmitted along the helix contour and that it is unlikely that DNA bending is responsible for the signaling. — P.D.