The Serratia nuclease is a non-specific endonuclease which cleaves single-and double-stranded RNA and DNA. It is a member of a large family of related endonucleases, most of which are dimers of identical subunits, with the notable exception of the Anabaena nuclease which is a monomer. In order to find out whether the dimer state of the Serratia nuclease is essential for its function we have produced variants of this nuclease which based on the crystal structure (Miller, M.D. and Krause, K.L. (1996), Protein Science 5, 24^33) were expected to be unable to dimerise. We demonstrate here that these variants, H184A, H184N, H184T and H184R, are monomers and have the same secondary structure, stability towards chemical denaturation and activity as the wild-type enzyme. This allows to conclude that the dimeric state is not essential for the catalytic function of the Serratia nuclease. In contrast, the S179C variant which is also a monomer shows little activity, presumably because this amino acid substitution changes the structure of the enzyme.z 1998 Federation of European Biochemical Societies.

In order to evaluate the pUR288-plasmid transgenic mouse model, utilizing the bacterial lacZ gene as the mutational target, radiation-induced mutagenesis was primarily analyzed in spermatogonial stem cells. A combined hydroxyurea (HU)-X-ray treatment protocol was used, known to sensitize dramatically the induction of mutations in endogenous genes. In the testes of untreated animals, a mutant frequency of 6.7 +/- 4.4 x 10(-5) was found. In animals treated with HU or X ray alone, moderate elevations were seen (factors of about 4 and 2 over untreated animal values). In testes of mice having received the HU + X-ray combination treatment, a mutant frequency of 63.0 +/- 36.1 x 10(-5) was found. The results obtained showed a good quantitative correlation between endogenous genes and the transgene, indicating the suitability of pUR288 transgenic mice for also efficiently recording radiation-induced genetic damage. Radiosensitization, seen in spermatogonial stem cells, was not observed in other studied organs such as spleen, brain, or lung.

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