| Summary: | <p>Abstract</p> <p>Background</p> <p>Manipulations of the DNA double helix during replication, transcription and other nucleic acid processing cause a change of DNA topology, which results in torsional stress. This stress is relaxed by DNA topoisomerases, a class of enzymes present in all domains of life. Negatively supercoiled DNA is relaxed by type IA topoisomerases that are widespread in bacteria, archaea and eukaryotes. In <it>Escherichia coli </it>there is conflicting data about viability of <it>ΔtopA </it>cells lacking topoisomerase I.</p> <p>Results</p> <p>In this study we sought to clarify whether <it>E. coli </it>cells lacking topoisomerase I are viable by using a plasmid-based lethality assay that allowed us to investigate the phenotype of <it>ΔtopA </it>cells without the presence of any compensatory mutations. Our results show that 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 <it>E. coli</it>, suggesting that the accumulation of torsional stress is, at least partially, responsible for the lethality of <it>ΔtopA </it>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 <it>ΔtopA </it>cells. However, we did not observe suppression of the <it>ΔtopA </it>phenotype by increasing the level of R-loop processing enzymes, such as RNase HI or RecG.</p> <p>Conclusions</p> <p>Our data show unambiguously that <it>E. coli </it>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 for the severe growth defect of cells lacking topoisomerase I, which is in contrast to the current literature. Potential reasons for this discrepancy are discussed.</p>
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