Lack of strand bias in UV-induced mutagenesis in Escherichia coli

Abstract

We have investigated whether UV-induced mutations are created with equal efficiency on the leading and lagging strands of DNA replication. We employed an assay system that permits measurement of mutagenesis in the lacZ gene in pairs of near-identical strains. Within each pair, the strains differ only in the orientation of the lacZ gene with respect to the origin of DNA replication. Depending on this orientation, any lacZ target sequence will be replicated in one orientation as a leading strand and as a lagging strand in the other orientation. In contrast to previous results obtained for mutations resulting from spontaneous replication errors or mutations resulting from the spontaneous SOS mutator effect, measurements of UV-induced mutagenesis in uvrA strains fail to show significant differences between the two target orientations. These data suggest that SOS-mediated mutagenic translesion synthesis on the Escherichia coli chromosome may occur with equal or similar probability on leading and lagging strands.

FIG. 1.

Insertion of lac operon into phage lambda attB site on E. coli chromosome in two orientations with regard to chromosomal replication oriC. The orientation in which the lac operon is transcribed in the same direction as the replication fork movement through the target is designated R, while the L orientation indicates lac transcription in the opposite direction (see arrows at attB). The arrows at oriC represent the two forks initiated at this site.

FIG. 2.

Shown are more detailed drawings of the replication fork configurations for R and L orientations for each of the six lacZ alleles used in this paper. lacZ102, lacZ106, lacZ107, lacZ108, lacZ110, and lacZ111 denote the lac alleles as originally present in strains CC102, CC106, CC107, CC108, CC110, and CC111 (6, 7), respectively. The dashed arrows indicate the lacZ coding strand (5′ → 3′). The nucleotides in the target sequences which may create pyrimidine dimers are indicated in bold and underlined.