Internal motion of deoxyribonucleic acid in chromatin. Nanosecond fluorescence studies of intercalated ethidium.

We have investigated the internal motions of DNA in a nucleosome core particle and chromatin by measuring the nanosecond fluorescence depolarization of intercalated ethidium. Assuming that the observed anisotrophy decay originates from the torsional motion of DNA, we have analyzed the dynamics of DNA in a nucleosome core particle and in chromatin in detail. The results suggest that DNA in a nucleosome core particle has a torsional rigidity similar to that of DNA in solution and that even at the point of the ionic bonds between DNA and a histone octamer the torsional motion of DNA is not completely inhibited. On the other hand, the dynamics of linker DNA in chromatin were found to reflect the overall structural state of the chromatin: the motion of linker DNA was suppressed as the structure of chromatin turned from an extended state to a condensed one. This indicates that, in solenoidal chromatin, nucleosome movements in chromatin are largely suppressed. Furthermore, the result may suggest that the torsional rigidity of linker DNA is increased as it is forced to bend in solenoidal chromatin.