Abstract. Uridine photohydrates (6-hydroxy-5,6-dihydrouridine) are the major products formed upon irradiation of the RNA bacteriophage R17 with ultraviolet light at 280 m1,u while cyclobutane-type pyrimidine dimerization does not occur to an appreciable extent. The suppression of dimerization may indicate that the RNA inside the phage is held in a rigid conformation in close contact with the phage protein. A value of 0.94 (+0. 10) uridine photohydrates per biological hit is obtained from a correlation of the fraction of phage surviving with the number of photohydrates formed as a function of ultraviolet dose. It is concluded that uridine photohydrates represent a major part of the lethal damage caused by ultraviolet light in bacteriophage R17. An efficient system for dark repair of uridine photohydration seems to be lacking in E. coli.It has been well documented1 that pyrimidine photodimers are an important part of the ultraviolet light-induced damage in double-stranded DNA. On the other hand, a correlation between the photoproducts formed in single-stranded nucleic acids and the biological lesions caused by ultraviolet light has not been possible. In particular, the role of photohydrates, the second major type of photoproducts formed from pyrimidine nucleotides, is not well understood. This may partially be attributable to the lack of reliable methods for the detection and determination of photohydrates in irradiated RNA and DNA. We have recently developed a simple procedure which allows the determination of uridine photohydrates (6-hydroxy-5,6-dihydrouridine, I) in irradiated RNA2 and of deoxycytidine photohydrates (6-hydroxy-5,6-dihydrodeoxycytidine) in DNA.3 Results from our laboratory and from others indicate that pyrimidine photohydrates may be formed in single-stranded synthetic polynucleotides,4 RNA,2 5 and DNA3 6 at rates comparable with, or even larger than, those for photodimerization. It has also become evident that uridine photohydrates may, under certain conditions, be significantly more stable in R17-RNA than in their monomeric form.7 Uridine photohydrates may therefore represent an important component of the biological damage caused by ultraviolet light in RNA. In this paper we report on the relationship between ultraviolet inactivation and uridine photohydration in the single-stranded RNA bacteriophage R17.

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