The excision of pyrimidine dimers from the DNA of mutant and wild-type strains of Ustilago.

UV-induced pyrimidine dimers were excised from the DNA of wild-type and four mutant strains of Ustilago maydis. Excision was partially dose dependent. The kinetics of excision differed in recombination deficient strains (rec 1 and rec 2) from those found in a recombination proficient radiation-sensitive strain (uvs 3). At fluences above 100 J-m-2 excision was saturated in uvs 3 but not in rec 1 or rec 2. Fluences above 300 J-m-2 started to saturate excision in wild-type. pol1-1, a temperature-sensitive DNA polymerase mutant, was excision proficient at both the permissive (22 degree) and restrictive (32 degree) temperatures. Wild-type cells were observed to excise CC before CT or TT in high dose experiments.