Efficiency of formation of pyrimidine dimers in SV40 chromatin in vitro

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Abstract
The efficiency of formation of pyrimidine dimers by 254 nm light was studied in mixtures of SV40 chromatin and DNA extracted from that chromatin. At high doses (beyond 380 J/m2), fewer dimers are formed in chromatin than in DNA for a given dose of radiation. This difference is about 10% as saturation with pyrimidine dimers is approached at 6840 J/m2. Conversely, at biologically repairable doses (up to 40 J/m2, less than 2 dimers/genome), significantly more dimers are produced in the chromatin than in the DNA. A maximum increase of about 50% occurs at doses producing 0.5-20 dimers/genome. With isolated nucleosomes from this chromatin, a maximum increase in dimer formation of 77% was observed. The increased dimer formation in the whole chromatin can be wholly accounted for in the nucleosome portion.