Volumne 7 Number 5 1979 Nucleic Acids Research Structure of plant nuclear and ribosomal DNA containing chromatin B.Leber and V.Hemleben Institut fiir Biologie II, Lehrstuhl f'ur Genetik, Universitat Tubingen, Auf der Morgenstelle 28, 7400 Tubingen, GFR Received 1 June 1979 ABSTRACT Digestion of plant chromatin from Brassica pekinensis and Matthiola incana with staphylococcus nuclease leads to a DNA repeat of 17518 and a core size of 140 base pairs. DNase I digestion results in multiples of 10 bases. Ribosomal RNA genes were studied as a model system for active plant chromatin because of their great redundancy and their high transcriptional activity in growing and differen- tiating tissues. The actively transcribed genes were identified by nascent RNA of ribosomal origin still attached to its matrix DNA. Hybridization techniques were used to demonstrate that even transcriptionally active gene sequences are present in nuclease generated chromatin subunits. Comparison of the DNase I kinetics of chromatin digestion with the amount of ribosomal RNA genes which is available for hybridization at the given times indicated that ribosomal RNA genes are digested, but not preferentially degraded by DNase I. INTRODUCTION Since the basic organization of chromatin into nucleosomes has been proved for eukaryotic organisms, the main interest is focussed on the question, whether structural changes are nescessary to permit the process of transcription. Several approaches have been employed to characterize transcriptionally active chromatin in animal systems. The fact that cDNA complementary to various messenger RNAs hybridizes to nuclease generated chromatin subunits indicated, that coding sequences are organized into nucleosomes (1,2,3). The observation, however, that limited DNase I digestion results in a preferential degradation of those sequences which are transcribed into tissue specific messenger RNAs, demonstrated, that transcriptionally active nucleosomes have an altered conformation which renders C) Information Retrieval Limited 1 Falconberg Court London Wl V 5FG England 1263