The extent of phenotypic expression of UV-induced str-r and str-d mutants in Escherichia coli B-r thy trp hcr

The effect of the preirradiation cultivation during tryptophan and thymine starvation on the extent of phenotypic expression ofstr-r andstr-d mutants ofEscherichia coli B/rthy − trp − hcr + was investigated. UV-light was used as a mutagenic factor. The preirradiation cultivation resulting in the inhibition of the total DNA synthesis did not practically influence the frequency of theStr-r andstr-d mutants. The extent of phenotypic expression was found to increase up to the 2nd hour of cultivation and the maximum phenotypic expression occurred only 4–5 h after the UV irradiation.