What is it about?
We investigated whether Escherichia coli cells can live without DNA topoisomerase I, the enzyme encoded by topA that helps control twisting stress in DNA. Using a plasmid-based assay, we examined cells before they had acquired compensatory mutations that can mask the real effect of losing this enzyme. We found that cells lacking topoisomerase I had an extreme growth defect and could not be maintained without selecting for compensatory changes. Extra topoisomerase III partly improved growth, but increasing enzymes that process RNA:DNA hybrids, also called R-loops, did not provide the same rescue.
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Why is it important?
This study helps clarify conflicting reports about whether E. coli cells lacking topoisomerase I are truly viable. Our results suggest that DNA twisting stress is an important part of the problem, while R-loops may not be the main cause of the severe growth defect in cells lacking topoisomerase I alone. At the same time, the stronger defects seen when RNase HI or RecG were also absent support the idea that RNA:DNA hybrid processing still matters in this background.
Perspectives
What stands out to us is that the experimental design let us look at the topA deletion phenotype without relying on strains that had already adapted through suppressor mutations. This made it possible to separate the effects of DNA topology from the effects of R-loop processing more clearly. The work is useful because it gives a more direct view of why loss of DNA topoisomerase I is so difficult for E. coli cells to tolerate.
Dr. Christian J Rudolph
Brunel University
Read the Original
This page is a summary of: On the viability of Escherichia coli cells lacking DNA topoisomerase I, BMC Microbiology, January 2012, Springer Science + Business Media,
DOI: 10.1186/1471-2180-12-26.
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