Figure 1.—

Isolation and analysis of suppressor mutants. (A) Approximately 10⁵ cells of the BY4742 rib5Δ deletion strain were plated on YPD and incubated for 6 days at 30°. Whereas the majority of the cells are unable to grow, some spontaneous suppressor mutants form colonies of various sizes. Similar analyses were performed for other strains and used to calculate the frequencies presented in Table 1. (B) Growth of purified rib5Δ* suppressor mutants on minimal medium containing the indicated concentrations of riboflavin. A riboflavin prototrophic wild-type strain (BY4742) was used as control. Nonsuppressed rib5Δ deletion strains require 20 mg/liter riboflavin (see Figure 3B).