Nucleic Acids Research, 2002, Vol. 30, No. 8 1799-1807
© 2002 Oxford University Press
Photoreactivation of UV-induced cyclobutane pyrimidine dimers in the MFA2 gene of Saccharomyces cerevisiae
School of Biological Sciences, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK
Photoreactive repair (PR) of cyclobutane pyrimidine dimers (CPDs) was mapped at nucleotide resolution in nucleotide excision repair (NER) proficient and deficient strains for the transcriptionally active and inactive MFA2 gene. Enhanced PR in the control region occurred in areas where no nucleosomes were present, particularly linker regions in the 
mating type. The presence of excision plus transcriptional activation alleviated this preference, causing repair in the regions that were linker and core in the mating type to be the same in this strain. Transcription had no effect on photoreactive repair in transcribed and downstream regions of MFA2, where similar rates were observed for specific CPDs in both strands. The presence of positioned nucleosomes in mating types revealed slow repair in the nucleosome core, with faster repair occurring at the 3' and 5' edges. These data support the evidence that CPDs are repaired quicker in nucleosome-free regions and at edges of nucleosomes. CPDs in the linker regions are repaired more efficiently in the transcriptionally inactive strains, suggesting that nucleosome movement associated with transcription of MFA2 hampers PR irrespective of the strand. Proficient NER influenced PR in the TATA and Mcm1 binding sites by enhancing it, particularly when transcription was activated.
* To whom correspondence should be addressed. Tel: +44 1792 295384; Fax: +44 1792 295447; Email: r.waters{at}swansea.ac.uk The authors wish it to be known that, in their opinion, the first two authors should be regarded as joint First Authors