The effect of caffeine on repair in Chlamydomonas reinhardtii. II. Interaction of repair systems.

Survival following UV-irradiation of the two repair-deficient strains of Chlamydomonas reinhardtii, UVSE5 and UVSE6, was not affected by caffeine. Since caffeine causes increased survival in strains of this organism having normal recombination, these two mutant strains are considered to be recombination-deficient. The double-mutant strains UVSE1--UVSE4, UVSE1--UVSE5, UVSE1--UVSE6, UVSE4--UVSE5, UVSE4--UVSE6 and UVSE5--UVSE6 were isolated. These strains were exposed to UV-irradiation and in all but UVSE4--UVSE5, survival of the double-mutant strain was much lower than for any single-mutant strain. These results indicate that the altered gene products in UVSE1, UVSE5 and UVSE6 mutant strains are associated with different recombination-repair mechanisms. All double-mutant strains were treated with caffeine following UV-irradiation. In all double-mutant strains containing a mutant USVE4 gene product, recombination repair was increased by caffeine. On the basis of the data obtained, a scheme is proposed for the involvement of multiple repair systems in repair following UV-irradiation in C. reinhardtii.