Table 2.

An rcbA plasmid suppresses the lethality caused by excess DnaA, regardless of the dnaA expression system, in mutants defective in DSB repair^a

Plasmid	Mean relative plating efficiency ± SD for strain (relevant genotype) and coresident plasmidb
	XL1-Blue (recA1) with pDS596 (dnaA)	SK002 (recB268::Tn10) with pDS596 (dnaA)	HMS174 (λDE3 recA1) with pKC596 (dnaA)
pACYC184	<1.1 × 10−3	<(1.9 ± 1.0) × 10−3	(2.2 ± 1.1) × 10−3
pMMF83 (rcbA)	0.9 ± 0.09	1.0 ± 0.1	0.8 ± 0.15
pMMF41 (hda)	1.2	0.8	1.45 ± 0.35
pMMF84 (datA)	1.1	1.0	0.7 ± 0.4
pMMF1 (dnaN)	0.8	0.9	1.4 ± 0.35

^aThe respective strains were coelectroporated with pACYC184, a plasmid encoding rcbA (pMMF83), or derivatives of pACYC184 carrying hda, datA, or dnaN and either pDS596 (50 ng) (dnaA⁺ under araBAD promoter control) or pKC596 (50 ng) (dnaA⁺ under the control of a bacteriophage T7 RNA polymerase promoter), as described in Materials and Methods. The strains were then plated onto antibiotic-supplemented LB medium lacking or containing either 0.5% (wt/vol) l-arabinose for strains containing pDS596 or 20 μM IPTG for strains containing pKC596. Incubation was done for 16 h at 37°C. The cotransformation efficiencies for these strains in the absence of an inducer ranged from 8.5 × 10⁴ to 2.3 × 10⁵ transformants per μg of pACYC184 DNA.

^bThe relative plating efficiency is the ratio of the number of colonies observed upon induced dnaA expression divided by the number of colonies in the absence of an inducer. The standard deviation was determined from at least three independent experiments. The shaded values were reported previously (29).