Abstract: For better understanding of Chlamydomonas reinhardtii DNA repair machinery, it is important to complete general phenotypical characteristics of repair-deficient mutant strains. So far these mutant strains were characterized on the basis of their response to UV light and MNNG. Now we have supplemented general characteristics with results obtained after X-ray irradiation. Up to date we have analysed mutants (most of them isolated in our laboratory) deficient in excision repair pathway (uvs12), recombination repair pathway (uvsE1, uvs10) or mutants without defined particular mechanism of DNA repair (uvs13, uvs14, uvs15).

The most sensitive strain after X-ray treatment was the strain uvs15, but this strain did not mutate. Moreover, the deficiency in pyrimidine dimer excision was proved in this strain. The mutant with such properties (excision deficiency with no or very reduced level of mutability) has not been described in heterotrophic organisms so far.

Strains uvsE1 and uvs10 with impaired recombinational repair pathway were also very sensitive to X-ray damage. These results indicate a posible role of recombination repair pathway in coping with DNA damage caused by X-ray irradiation. Moreover, we observed increased frequencies of streptomycine-resistant mutations in the strains. It suggests that recombination repair is an error-free process.

The strain uvs14 was the most resistant one of all UV-sensitive strains used. In spite of the fact that the strain uvs14 manifests a muatator phenotype (higher levels of spontaneous and induced mutability), we did not observed significantly increased uvs14 mutability after X-ray treatment.