Base mismatches and mutagenesis: how important is tautomerism?

Tautomerism of nucleotides is an attractive model to explain transitional mutations; the structure of the mismatched base pairs in their enol or imino forms do not distort the DNA double helix. However recent structural data suggest that the mismatched nucleotides are in their normal keto and amino forms. 'Wobble' and other base pairings (for transversions) have C1'-C1' distances close to those found in the classical Watson-Crick base pairs. Kinetic studies show substrates are in their major tautomeric forms in their transition states. This suggests tautomerism may not be important for substitution mutations.