We generated a rnhA recG proB::rnhA + strain in which the recG deletion was covered by pJJ100 (pRC7 recG + ). As shown in Figure 3A, only very small

white colonies were observed after incubation for 48 h on LB agar without arabinose. These white colonies are formed due to the leakiness of the araBAD promoter. In contrast, on LB agar with moderate arabinose concentrations robust segregation of blue and white colonies was observed, with the white colonies being as healthy as the blue. Thus, expression of the integrated rnhA construct can be regulated by the presence or absence of arabinose. Figure 3 The lethality of ΔtopA cells is not suppressed by increased levels of RNase HI. (A) Expression GDC-0973 cost of a P araBAD rnhA construct integrated into the chromosome can be regulated by different arabinose concentrations. The expression level is high enough to suppress the synthetic lethality of rnhA recG cells. (B) Expression from the integrated P araBAD rnhA construct does not suppress the lethality of ΔtopA cells. The P araBAD rnhA construct has been integrated into a rnhA + background. Thus, expression of the construct will produce RNase HI in addition to the regular rnhA locus. (C) Expression from the integrated P araBAD rnhA construct does not improve growth of cells in which the ΔtopA

defect is partially suppressed by overexpression of DNA topoisomerase III. The image for AS1066 was reproduced from Figure 2 for comparison. Please note that incubation and image capturing procedures are standardised to allow comparison of colony find more sizes To test whether increased MG-132 price levels of RNase HI can suppress the lethality of topA strains

we integrated our proB::rnhA + expression construct into an rnhA + background. Thus, any expression from our integration construct will be in addition to the expression from the native rnhA gene. We then introduced our topA::apra allele, covering the deletion with the pRC7 topA plasmid. However, growth of this strain in medium with moderate (data not shown) or high arabinose concentrations did not lead to formation of white colonies (Figure 3B). Since we did not directly measure the concentration of RNase HI in cells we cannot exclude the possibility that the levels in our expression constructs are not high enough for suppression of the ΔtopA phenotype. We therefore wanted to test the expression of rnhA in a system that might be more sensitive for low expression levels. It was observed before that the co-expression of both rnhA and topB resulted in a synergistic suppression of the topA phenotype [14]. We therefore wanted to know whether the expression of rnhA from our integration construct would increase the suppression of the observed topB overexpression. To test this we transformed our ptopA/ΔtopA ΔproB::rnhA + background with the topB expression plasmid. However, co-expression did not lead to an increase in the size of the white colonies. If anything a mild reduction of viability is observed (Figure 3C).