The nucleosome subunit of chromatin consists of DNA folded around a histone core as a 1.8-turn left-handed solenoid. The crystal structure1 of the nucleosome core particle revealed that it has a dyad symmetry axis and that the minor helix groove faces outwards from the protein core. Richmond et al.1 noticed that the path traversed by the helix has severe bends at sites approximately one and four helix turns from the dyad axis. We have developed two photochemical methods to study the structure of DNA, and in particular that wrapped around the nucleosome core. One method depends on the sensitization of singlet oxygen production by an eosin analogue. We have monitored the rate at which excited state oxygen diffuses into contact with DNA base planes, and find that it attacks the nucleosome with high specificity. We have also mapped the DNA binding of the intercalating dye methylene blue, and conclude that it binds to the same sites accessible to oxygen by diffusion. On the basis of these results we suggest that the DNA in the nucleosome is bent or kinked at two sites, 1.5 helix turns from the dyad axis.
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