PREFERENTIAL REPLICATION DEPENDENT MUTAGENESIS IN THE LAGGING DNA STRAND IN ESCHERICHIA-COLI

The mutation frequencies attributable to -1 frameshift or one-base sub stitution in the structural genes coding for resistance to chloramphen icol (Cm) and tetracycline (Tc) were followed over several cycles of D NA replication, and found to differ several-fold, depending on the ori entation of the gene on the plasmid with respect to the direction of ( unidirectional ColE1-type) replication. The mutation frequency was hig her when the reporter gene was present in the plasmid in the same orie ntation as the direction of the origin, i.e., when the transcription t emplate is the lagging daughter strand, than when the gene was inserte d in the opposite orientation. This significant difference in reversio n frequencies of genes with different polarities was demonstrated only for a brief period of cell growth (several cycles of replication) aft er induction of the dnaQ49 mutator, but was not observed when an incre ased number of replication cycles, was permitted, most probably due to fixation of the mutation into both strands. The mutated intermediate DNA which possesses a misaligned basepair in the Cm gene was demonstra ted to be replicated into two progeny DNA molecules; one is the chlora mphenicol-resistant (Cm-R) DNA synthesized from the template strand ha ving the mutation and the other is the Cm-S DNA from the template stra nd without mutation. Our results suggest that replication-dependent mu tagenesis may occur preferentially in the lagging strand.