TABLE 1.

Leading- and lagging-strand mutation frequencies in dnaN⁺ and dnaN159 strains

Straina	dnaN allele	Orientation of lacZ[GGG→ GAG] alleleb	Mutants/106 CFUc
			lacZ[GGG→ GAG]→ Lac+		Rifr
			Mutation frequency	Effect (n-fold) (leading/lagging)	Mutation frequency	Effect (n-fold) (leading/lagging)
JL100	dnaN+	Leading	0.60 ± 0.13	2.9	4.87 ± 1.77	1.3
JL101	dnaN+	Lagging	0.21 ± 0.05		3.74 ± 0.22
JL102	dnaN159	Leading	1.40 ± 0.47	0.9	7.31 ± 1.95	1.1
JL103	dnaN159	Lagging	1.59 ± 1.29		6.59 ± 1.23

^aStrains JL100 to JL103 are derived from strains EC3144 and EC3138 (10) and differ only in their respective dnaN alleles (dnaN⁺ or dnaN159), as well as the orientation of the lacZ[GGG→ GAG] allele (leading or lagging) integrated at attB. Their remaining genotype is ara Δ(lac-proB)_XIII attB::lacI⁺Z[GGG→ GAG]Y⁺A⁺ tnaA300::Tn10 ΔmutL::cat.

^bThe lacZ[GGG→ GAG] allele is integrated at attB so that the coding strand is replicated as part of the leading strand (Leading) or as part of the lagging strand (Lagging).

^cGrowth of strains bearing the lacZ[GGG→ GAG] allele on minimal media containing lactose as the sole carbon source requires a “true” reversion of the lacZ mutation, as no other mutation confers a Lac⁺ phenotype. Mutation frequencies were measured as described previously (10). The results shown represent the averages of at least three independent determinations ± the standard deviation.