DIFFERENT MODES OF LOSS OF PHOTOREVERSIBILITY OF MUTATION AND LETHAL DAMAGE IN ULTRAVIOLET-LIGHT RESISTANT AND SENSITIVE BACTERIA

Loss of photoreversibility of UV-induced mutation to streptomycin resistance and lethal damage in an hcr+ strain of Escherichia coli occurs during the first 20 min of postirradiation incubation and is unaffected by absence of tryptophan with an amino acid-requiring substrain or by absence of thymine with a thymine-requing substrain, but is blocked by dinitrophenol. The initial postirradiation replication of DNA which begins after 40 min of incubation is significant in the mutation process since expression of streptomycin resistance (measured in streptomycin-containing agar medium blocking protein synthesis) does not occur unless DNA replication is allowed before plating and is thymine-dependent. Loss of photoreversibility of streptomycin-resistant mutations and lethal damage in an hcr- strain of E. coli begins after 20 min of incubation and requires 80 min for completion and is tryptophan-dependent and thymine-dependent. Unlike streptomycin-resistant mutation, loss of photoreversibility of suppressor mutations in a tryptophan-requiring strain is correlated with DNA replication even in hcr+ bacteria and is amino acid- and thymine-dependent in hcr- bacteria. © 1969.