Demonstration of the production of frameshift and base-substitution mutations by quasipalindromic DNA sequences.

The in vivo production of frameshift and base-substitution mutations predicted as a consequence of the metabolic processing of misaligned quasipalindromic DNA sequences has been confirmed. Spontaneous frameshift mutations of the T4 rII gene that had been genetically mapped to quasipalindromic DNA sequences were sequenced. Some of the mutant sequences are exactly those predicted by a mutational model based on misaligned quasipalindromes. Furthermore, these sequences are distinct from those predicted by the classical frameshift model based on misaligned repeated sequences. The rII frameshift mutant sequences reported here result from the deletion of a specific base or bases that would remain looped out should the quasipalindromes assume a hairpin secondary structure. One hairpin predicted not only the deletion of two bases (a frameshift) but the concomitant production of nearby but noncontiguous base substitutions. The substitution of as many as three bases as well as the frameshift were predicted to arise as a consequence of a single mutational event in the palindrome. Two independent examples of the predicted deletion frameshift were found among the small sample of sequenced spontaneous frameshifts examined and both were associated with the predicted transversion and transition base substitutions.