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Different compounds known to inhibit dark repair in bacterial and mammalian systems have been tested for a similar effect in the yeast Schizosaccharomyces pombe. A marked inhibition of growth by these different drugs indicated that these were taken up by the cell, yet none except caffeine reduced the colony forming ability when present in the post-irradiation plating medium. The response of 22 different radiation sensitive mutants to the presence of caffeine in the plating medium was tested to investigate the probable number of pathways for repair of radiation damage in this organism. In some of the genetic crosses among single gene mutants showing a differential caffeine-response, supersensitive double mutants were obtained. Strains obtained either by additional crosses or by further mutagenic treatment of the supersensitive double mutants did not show a highly significant increase in radiation sensitivity. These observations indicate that the presently available radiation sensitive mutants represent defects in only two major pathways for repair of radiation damage in the wild type S. pombe.

Non-photoreactivable endonuclease V-sensitive sites have been detected in the DNA of wild type bacteriophage T4 irradiated with near UV light (320 nm). Such sites were not detected in the DNA of (a) wild type T4 irradiated with far UV (254 nm) or (B) in T4 mutants in which non-glucosylated 5-hydroxy-methylcytosine (5HMC) or cytosine replaces glucosylated 5HMC normally present in T4, irradiated with 320 nm or 254 nm light. Although the non-photoreactivable sites accounted for approximately 50% of the endonuclease V-sensitive sites in the DNA of glucosylated T4 irradiated with near UV, there was very little difference in the sensitivities of T4 containing glucosylated 5HMC, non-glucosylated 5HMC and cytosine to near UV (313 nm). We propose that the photoproduct responsible for the non-photoreactivable, but endonuclease V-sensitive, sites in glucosylated DNA is formed from glucosylated 5HMC and that a similar photoproduct is formed from non-glucosylated 5HMC or cytosine in the appropriate phage strains. We further propose that the glucosylated 5HMC photoproduct is non-photoreactivable whereas the cytosine and non-glucosylated 5HMC photoproducts are photoreactivable and are therefore possibly cyclobutane dimers.