TABLE 3.

Differential mutator effects of ΔholE and ΔholE::hot: Lac⁺ or Rif^r mutant frequencies in mismatch repair-defective mutL strains^a

lac allele from strain	Mutation scored	No. of mutants (per 106 cells)			Mutator effect (fold)
		holE+	ΔholE	ΔholE::hot	ΔholE/ holE+	hot+/ holE+
Allb	Rifr	0.44	0.71	0.7	1.6	1.6
CC101	A · T→C · G	0.0036	0.0027	0.0046	0.75	1.3
CC102	G · C→A · T	0.44	0.45	0.79	1	1.8
CC103	G · C→C · G	0	0	0
CC104	G · C→T · A	0.009	0.038	0.017	4.2	1.9
CC105	A · T→T · A	0.0037	0.015	0.004	4	1
CC106	A · T→G · C	0.17	0.55	0.36	3.2	2.1
FC40c	lac (−1) FSd	0.096	0.093	0.27	0.97	2.8

^aStrains are mutL::Tn5 derivatives of KA796 (holE⁺), NR16787 (ΔholE), and NR17012 (ΔholE::hot eda-51::Tn10) or NR17013 (ΔholE::hot zeb-3190::Tn10) (Table 1) and contain the F′(pro lac) episome from strains CC101 through CC106 (Table 1), permitting measurement of the indicated, specific lac reversion frequencies. The eda or zeb transposon insertions did not affect the hot mutator effect (results not shown). For each strain with each F′(pro lac), 22 independent LB cultures were grown overnight at 37°C. The 22 cultures were derived from multiple isolates (transductants) for each strain. The frequencies of lac⁺ revertants and of Rif^r mutants were determined as described in Materials and Methods. The mutant frequencies were analyzed by Prism software, and statistically significant differences between hot⁺ and ΔholE or hot⁺ and ΔholE::hot strains (mutator effects) were assessed using nonparametric tests. Differences with P values of <0.05 are shown in boldface type.

^bThe Rif^r data reflect the median value of eight independent experiments, each comprised of 22 cultures.

^cThis strain was tested in the mismatch repair-proficient background.

^dFS, frameshift mutation.