Mutagenesis by transient misalignment.

Based upon a consideration of two mutational hot spots produced during DNA synthesis by a eukaryotic DNA repair polymerase, we suggested that certain base substitution errors result not from direct miscoding but from correct coding by a transiently misaligned template-primer (Kunkel, T. A., and Alexander, P. S. (1986) J. Biol. Chem. 261, 160-166). This model, which we called dislocation mutagenesis, has been directly tested. Introducing a single, phenotypically silent G—-A base change into the template switches the base substitution specificity at the immediately adjacent hot spot, a T residue, from T—-G transversions to T—-A transversions. The cumulative change in frequency, represented by the disappearance of the T—-G events and the appearance of the T—-A events, is greater than 300-fold. These data demonstrate that during DNA synthesis in vitro, a base at one position can code a mutation at another position. This mechanism can operate over greater distances to produce complex mutations as well. We present one example in which a 123-base deletion containing three base changes at one end of the deletion can be precisely explained by transient misalignment. It remains to be established whether mutagenesis by dislocation operates in vivo to produce biologically significant changes in genetic information.