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 exonucleolytic 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.