p285

E coli without topoisomerase I

A Stockum et al., 

On the viability of Escherichia coli cells lacking DNA topoisomerase I 

BMC Mol Biol 12 26 (2012)

Cells lacking topoisomerase I show an extreme growth defect and cannot be cultured without the accumulation of compensatory mutations. This growth defect can be partially suppressed by overexpression of topoisomerase III, the other type IA topoisomerase in E. coli, suggesting that the accumulation of torsional stress is, at least partially, responsible for the lethality of ∆topA cells. The absence of RNase HI strongly exacerbates the phenotype of cells lacking topoisomerase I, which supports the idea that the processing of RNA:DNA hybrids is vitally important in ∆topA cells. However, we did not observe suppression of the ∆topA phenotype by increasing the level of R-loop processing enzymes, such as RNase HI or RecG.

 Unambiguously that E. coli cells are not viable in the absence of DNA topoisomerase I without the presence of compensatory mutations. Furthermore, our data suggest that the accumulation of R-loops is not the primary reason.