Classification of Δsmc suppressor mutants.

The twenty suppressors were grouped in seven different classes based on phenotypic analyses. Colony size was estimated after 72h at 23°C on rich medium: Colony size similar to wild type is large (L), similar to Δsmc is small (S), and intermediate is medium (M). Plating efficiencies were determined from cells in exponential growth phase. Survival assays on mitomycin C (MC), coumermycin A1 (Coum), and nalidixic acid (Nal. Acid) were performed on rich medium at 23°C with the indicated concentrations. The ratio between the numbers of colonies obtained in each condition relative to the number of viable cells on rich medium at 23°C was calculated. The value in the table corresponds to the means of the ratio for at least two independent experiments per strains. Five suppressors formed colonies almost as large as wild type colonies. Three of these (class II: s35, s36, s37) were able to grow at 51°C but still demonstrated a high sensitivity to coumermycin A1, unlike the remaining two (class VII: s33, s39). Among suppressors forming small colonies, five were unable to grow at 51°C on rich medium and were separated into two classes to their resistance (class III: s38, s40, s41) or sensitivity (class V: s43, s49) to gyrase inhibitors. The final suppressors (nine of the original twenty) grew at 51°C, formed small colonies, were resistant to coumermycin A1 and showed either an intermediate sensitivity (class IV: s44, s46, s47, s48, s52, s53, s54) or high resistance (class VI: s42, s55) to nalidixic acid. Suppressor s34 showed nearly the same phenotype as the suppressors belonging to the class VI, although it appeared earlier and formed larger colonies on plates, earning its separation from the other suppressors in the class I.Classification of Δsmc suppressor mutants.