Influence of chromatin folding on transcription initiation and elongation by RNA polymerase III

Nucleosomes were assembled onto either closed circular plasmids containing a single Xenopus 5S RNA gene or a linear tandemly repeated array of Lytechinus 5S RNA genes. Both chromatin templates were found to vary in their extent of compaction, depending upon the type and concentration of cation in solution. Compaction of these chromatin templates led to a significant inhibition of both transcription initiation and elongation by RNA polymerase III. Thus, the transcriptional repression observed after incorporation of genes into chromatin depends not only on occlusion of the promoter elements through direct contact with histones but also on compaction of nucleosomal arrays which occurs under the conditions of the transcription reactions.