Transcribed chromatin

In eukaryotes, DNA that is transcribed is packaged first into nucleosomes and then into chromatin fibres. How does transcription proceed through chromatin? Studies of transcription through nucleosomes in vitro suggest that the intracellular environment may provide factors which alleviate the inhibitory effect that nucleosomes have on transcription, possibly via positive supercoiling induced by the migrating polymerase. Stable changes in nucleosome structure have been correlated with transcriptionally active chromatin, but the precise mechanism by which RNA polymerase transcribes through nucleosomal DNA remains unknown.