Modulation of cyclobutane pyrimidine dimer formation in a positioned nucleosome containing poly(dA.dT) tracts.

We have used a defined-sequence nucleosome to concomitantly investigate the generation and location of DNA lesions in nucleosomes and their influence on nucleosome positioning (translational and rotational setting). A 134 bp HISAT sequence from the yeast DED1 promoter, containing a polypyrimidine region (40 bp) with a T6-tract, two T5-tracts, and a T9-tract, was reconstituted in nucleosomes with a defined rotational setting. T-tracts adopt unusually rigid DNA structures in solution ("T-tract structure") and are hot spots of cyclobutane pyrimidine dimer (CPD) formation by UV light (254 nm). DNA was irradiated with UV light before or after reconstitution. The CPD yields and distribution were analyzed by cleavage with T4 endonuclease V. The rotational setting of nucleosomal DNA was characterized by DNase I digestion. With the exception of one T5-tract (1T5), the T6-, the 2T5-, and the T9-tracts formed T-tract structure in solution. T-tract structure was lost upon folding in nucleosomes, demonstrating a dominant constraint of DNA folding in nucleosomes over that of T-tract structure. CPD formation was strongly modulated by the nucleosome structure, but the CPD distribution differed from that reported for mixed-sequence DNA. CPD formation in the nucleosome had no effect on the rotational setting of nucleosomal DNA, but the rotational setting was affected when nucleosomes were assembled on damaged DNA. The toleration of DNA distortions imposed by CPDs in nucleosomes may have important implications for the recognition and repair of these damages in chromatin.