Mechanics of chromatin template activation. Physical evidence for destabilization of nucleoproteins by polyanions.

Thermal denaturation profiles were used to quantitate the destabilizing activity of five classes of polyanions in an effort to explain previously reported differences in activity among apparently similar polymers. This physical assay for polyanion action on chromatin is free of the biochemical complications associated with template assay systems. Most polyanions tested caused a thermal destabilization of the normal nucleohistone complexes of rat thymus chromatin and exposed free or weakly complexed DNA. Polyphosphates dissociated chromatin even when their net charge was as low as 6, and activity per unit of weight increased with chain length only for polymers containing less than about 20 phosphate units; pyrophosphate was inactive. Two polymers which had no influence on chromatin had a low negative charge density. Some, such as polycytidlate and denatured DNA, possessed a high charge but caused only minor changes in the shape of denaturation profiles and exposed little or no free DNA. The highly active nucleic acids, ribosomal RNA and polyguanylate, were distinguishable from inactive nucleic acids by having more secondary structure and potentially less steric hindrance for anionic interaction with chromatin. There was evidence that some polyanions dissociate certain histones before others. Although data obtained show that some histones may be redistributed from chromatin onto added double-stranded DNA in a medium containing urea, this does not appear to interfere significantly with the interpretation of typical thermal denaturation profiles of chromatin.