In Vitro Processing of Heteroduplex Loops and Mismatches by Endonuclease VII

Endonuclease VII is a Holliday-structure resolving enzyme ofphage T4 which cleaves at junctions of branched DNAs and atmispairings. In extension of these findings we report the following:i) Endonuclease VII can discriminate between a large heteroduplexloop and a TT mismatch arranged in tandem, 6 nt distant fromeach other, in the same heteroduplex molecule. The enzyme cleavestwo nucleotides 3' from the base of the loop or the TT mismatch.ii) Similar to its reactions with mismatches cleavage of heteroduplexloops by endonucleave VII can also initiate correction of perfectdouble-strandedness by T4 DNA polymerase and T4 DNA-ligase invitro. Loops of 8 nt and 20 nt were repaired efficiently. iii)For the first time endonuclease VII cleavage sites were alsomapped in single-stranded DNA if it was part of the 20-nt loop.This suggests that looping of single-stranded DNA can induceformation of secondary structures, which are recognizable byendonuclease VII.