FIG. 4.

Spontaneous mutation frequencies in dnaN⁺ and dnaN159 strains expressing either Pol I or the Klenow fragment of Pol I. All strains used were derivatives of CJ278 [Δ(gal-bio) thi-1 relA1 spoT1 ΔpolA::kan] and expressed either the polA⁺ allele, the Klenow fragment of Pol I, or the polA-exo mutant allele from an F′ episome (18). The strains examined were RM137, dnaN⁺ (F′ polA⁺); RM138, dnaN159 (F′ polA⁺); RM139, dnaN⁺ ΔumuDC595::cat (F′ polA⁺); RM140, dnaN159 ΔumuDC595::cat (F′ polA⁺); RM141, dnaN⁺ (F′ Klenow); RM142, dnaN159 (F′ Klenow); RM143, dnaN⁺ ΔumuDC596::ermGT (F′ Klenow); RM144, dnaN159 ΔumuDC596::ermGT (F′ Klenow); RM145, dnaN⁺ (F′ polA-exo mutant); RM146, dnaN159 (F′ polA-exo mutant); RM147, dnaN⁺ ΔumuDC596::ermGT (F′ polA-exo mutant); RM148, dnaN159 ΔumuDC596::ermGT (F′ polA-exo mutant). The mutation frequency was calculated following growth at 30°C by dividing the number of Rif^r colonies by the total number of viable cells, as described previously (29, 38, 40). Mutation frequencies are expressed relative to the dnaN⁺ polA⁺ strain (9.7 ± 7.5 Rif^r CFU/10⁹ total viable cells), which was set equal to 1.0, and are the averages of at least five independent determinations. The error bars represent the standard deviations. Abbreviations: +, polA⁺, dnaN⁺, or umuD⁺C⁺, as indicated; exo⁻, Pol I exo mutant; K, Klenow fragment; 159, dnaN159; Δ, ΔumuDC596::ermGT or ΔumuDC595::cat.