Dependence of the broth effect and the phenomenon of mutation frequency decline on dose of the applied UV radiation was investigated in the strain Escherichia coli B/r Hcr+ thy trp. Reversions to Trp+ were followed. The degree of the broth effect and the mutation frequency decline is minimal within the range of UV doses corresponding to a survival of cells lower than 10(-1). In connection with the two effects, excision of thymine dimers, initiation of synthesis, synthesis and degradation of DNA were also investigated. It was found that stimulation or inhibition of an inaccurate postreplication repair mechanism, rather than inhibition or stimulation of excision of thymine dimers, are responsible for the broth effect and the mutation frequency decline, respectively.

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