TABLE 3.

Mutation rates^a

Strain	Genetic background	Relevant genotype	Expt no.	Rate of mutationb (10−10) phenotype indicated
				Nalr	Strr	Arg+
JC11450	JC11450	xonA+ xseA+ recJ+ mutL+	1	1.5	NAc	12
			2	1.2	NA	11
			3	3.7	NA	45
SMR3488	JC11450	ΔxonA300::cat recJ284::Tn10 ΔxseA18::amp	1	<1.6d	NA	44
			2	<1.2d	NA	36
			3	3.0	NA	59
SMR3524	JC11450	mutL211::Tn5	1	580	NA	740
			2	700	NA	430
			3	700	NA	220
FC40	FC40	xonA+ xseA+ recJ+ mutL+	3	5.0	1.3	NA
			4	4.7	0.83	NA
			5	3.6	0.82	NA
SMR3481	FC40	ΔxonA300::cat recJ284::Tn10 ΔxseA18::amp	3	2.4	0.74	NA
			4	<2.9d	<1.0d	NA
			5	<3.8d	<1.1d	NA
SMR3404	FC40	mutL211::Tn5	3	1,100	91	NA
			4	1,000	40	NA
			5	1,200	53	NA

^aFor each strain and experiment, the number of cultures was 25. 

^bNumber of mutations per cell per generation. Nalidixic acid-resistant (Nal^r) and streptomycin-resistant (Str^r) colonies were selected on Luria-Bertani-Herskowitz (LBH) plates (see, e.g., reference 36) supplemented with 40 μg of nalidixic acid or 100 μg of streptomycin per ml. Arginine prototrophs (Arg⁺) were selected on minimal M9 plates (21) with the addition of 0.1% glycerol and 5 μg of the appropriate amino acids per ml. Mutants were scored after ca. 24 (LBH) or 72 h (M9) of incubation at 37°C. Mutation rates were calculated by the method of the median as modified by von Borstel (38). 

^cNot applicable (JC11450 is Str^r and FC40 is Arg⁺). 

^dIn these cases more than half of the cultures produced no mutant colonies and the mutation rate was calculated using a median of <1; the rates are thus overestimates and are preceded by “<.”