Figure 1.

Maintenance of cell viability in the absence of DNA topoisomerase I. (A) Effect of the deletion of topA. The plate photographs shown are of synthetic lethality assays. These, and similar assays reported in subsequent Figures, are described in detail in Materials and Methods. The relevant genotype of the construct used is shown above each photograph, with the strain number in parentheses. The fraction of white colonies is shown below with the number of white colonies/total colonies analyzed in parentheses. (B) Abortive growth of a plasmid-free colony from panel A iv after re-streaking on minimal medium. Large colony variants indicate the rapid accumulation of suppressor mutations in a topA single mutant. (C) Effect of ΔtopA75 on viability