A chromatin-like material was prepared from isolated nuclei of Saccharonayces cerevisiae. This chromatin fraction contains a full complement of yeast histones as revealed by acrylamidegel electrophoresis. Yeast histones were found to resemble those of rat liver cells in electrophoretic properties except for the almost complete absence of histone F1. Yeast chromatin is only poorly transcribed by the isolated yeast RNA polymerases A and B and was found to contain endogenous RNA polymerases which are activated by the addition of salts. Salt causes a removal of DNAassociated proteins from the chromatin, allowing bound RNA polymerase molecules to extend already initiated RNA chains. Some evidence was obtained indicating that different proteins have to be removed to allow the different endogenous polymerases to proceed with transcription. It is concluded that the properties of yeast chromatin studied, in particular the histone composition and the transcription capacity, are very similar to those of chromatin preparations from animal cells.Due to the complex structural organization of the genetic material in higher organisms some of the reactions involved in gene expression have no analogue in bacteria. This calls for a simple eukaryote which could serve as a model system to solve basic problems in this field. The unicellular eukaryote, Saccharomyces cereuisiae, is for many reasons the model of choice. Although the amount of genetic material in these cells is less than 1j100 of most animal cells [1], the existence of so many similarities with regard to structural and functional properties of the transcription system is striking. Earlier work has already indicated that the synthesis of ribosomal RNA in ycast follows essentially the same principles as that of more complex eukaryotic cells [2-41. Also the RNA polymerases of yeast closely resemble those of animal cells [5,6]. With the work reported in this paper we will add one more important characteristic demonstrating the similarity between yeast and higher cells : a chromatin-like material prepared from yeast nuclei, hereafter referred to simply as chromatin in the tcxt, is shown to contain histones which have electrophoretic properties similar to those of liver histones. The chromatin preparation furthermore functions as template for purified ycast RNA polymerases and contains endogenous RNA polymerases which can be activated by the addition of salt.Enzyme. RNA polymerase or nucleosidetriphosphate : RNA nucleotidyltransferase (EC 2.7.7.6). METHODS Strain and Growth ConditionsA diploid, wild-type strain of Saccharomyces cereuisiae was used in this work. Cells were grown a t 30 "C with continuous shaking in a medium consisting of 2O//, peptone, 1 yeast extract and 2O//, glucose to a density of about 5 x lo7 cells per ml. They were harvested by centrifugation and washed twice with distilled water. Isolation of Yeast NucleiThree alternative procedures were used for the isolation of nuclei. Two of these involved the prior conversion of cells to spheroplasts, which were the...

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