LEADING-STRAND SPECIFIC SPONTANEOUS MUTATION CORRECTS A QUASIPALINDROME BY AN INTERMOLECULAR STRAND SWITCH MECHANISM

imperfect inverted repeats or quasipalindromes can undergo spontaneous , often complex mutational events that correct them to perfect palindr omes. Two models that depend on the quasipalindrome providing a templa te for a specific mutational event have been described to explain this mutation: an intramolecular anal an intermolecular strand switch mode l. A 17bp quasipalindrome containing a -1 deletion within the chloramp henicol acetyl transferase (CAT) gene in plasmid pJT7 undergoes a spon taneous +1 frameshift mutation that creates a perfect inverted repeat and a Cm-r phenotype. By analyzing this mutation frequency in two plas mids that contain the CAT gene in either orientation with respect to t he origin of replication, we show that the specific frameshift occurs preferentially in the leading strand during DNA replication. Due to th e availability and proximity of the lagging strand template as a singl e strand during replication of the quasipalindrome in the leading but not lagging strand, we suggest that the specificity for the leading st rand correction is due to a leading strand specific intermolecular str and switch rather than an intramolecular strand switch. To test this h ypothesis, we have designed a genetic selection to detect a leading st rand intermolecular strand switch. This selection utilizes asymmetric quasipalindromes, one of which contains tw central stop codons. When c loned into the CAT gene in pJT7, reversion to Cm-r requires inversion of the stop codons and addition of a +1 frameshift to correct the read ing frame. The inversion of the central stop codons, which is predicte d by an intermolecular but not an intramolecular strand switch, occurs concomitant with the specific correction of the original 17 bp quasip alindrome. Inversion of an asymmetric center can also be demonstrated when not under selective pressure using a quasipalindrome lacking cent ral stop codons. These results are consistent with the correction of a quasipalindrome occurring predominantly by an intermolecular strand s witch during replication of the leading strand. (C) 1997 Academic Pres s Limited.