EFFECTS OF BASE MISMATCHES ON JOINING OF SHORT OLIGODEOXYNUCLEOTIDES BY DNA LIGASES

The requirement for Watson-Crick base pairing surrounding a nick in du plex DNA to be sealed by DNA ligase is the basis for oligonucleotide l igation assays that distinguish single base mutations in DNA targets, Experiments in a model system demonstrate that the minimum length of o ligonucleotide that can be joined differs for different ligases, Therm us thermophilus (Tth) DNA ligase is unable to join any oligonucleotide of length six or less, while T4 DNA ligase and T7 DNA ligase are both able to join hexamers, The rate of oligonucleotide ligation by Tth DN A ligase increases between heptamer and nonamer. Mismatches which caus e the duplex to be shortened by fraying, at the end distal to the join , slow the ligation reaction, In the case of Tth DNA ligase, mismatche s at the seventh and eighth position 5' to the nick completely inhibit the ligation of octamers. The results are relevant to mechanisms of l igation.