TWIST CONSTRAINTS ON LINKER DNA IN THE 30-NM CHROMATIN FIBER - IMPLICATIONS FOR NUCLEOSOME PHASING

Previous work has shown that nucleosome repeat lengths, and hence link er DNA lengths, are preferentially quantized to a set of values differ ing by integral multiples of the helical twist of DNA. An explanation was proposed in which this preferential quantization is due to twist c onstraints on linker DNA arising from nucleosome-nucleosome interactio ns in folded chromatin. Here we report the results of a study, using e thidium intercalation, designed to test whether twist constraints do i ndeed exist. Electron microscopy reveals that ethidium intercalation c auses decondensation of dinucleosomes. Direct measurement of the free energy of intercalation by fluorescence spectroscopy reveals competiti on between chromatin folding and ethidium intercalation. Results from other laboratories establish that these effects of ethidium are due to ethidium-induced changes in the twist of linker DNA, and not to a var iety of other possible effects. We conclude that twist constraints on linker DNA do exist. These may explain the observation of preferential ly quantized linker DNA lengths. Implications of these results for mec hanisms of nucleosome phasing and the mechanisms of drug action are di scussed.