Preparation of Chromatin.

Four of the six chromatin preparation methods examined here are shown to produce chromatins whose template properties differ radically from one another. Comparing the template-activity profiles obtainable with endogenous (i.e. chromatin-bound) and exogenous form B rat liver RNA polymerases across a range of salt concentrations, it appears that only one of these chromatins resembles isolated nuclei at all closely. A study of RNA product size on formamide gradients further reveals that only in this single case is there any sign of product-chains larger than 28-S rRNA. However, all these chromatins contain considerable exonucleotytic RNase activity, more marked at low than at high ionic strengths. In addition, prolonged (30-s) shearing is shown to cause drastic changes in the template properties of chromatin, as well as solubilizing it by reduction of fragment size. [ABSTRACT FROM AUTHOR]