The separation of transcriptionally engaged genes.

We have developed a method for the separation of transcriptionally engaged chromatin from inactive genes as well as from active genes which are not being transcribed. This approach is dependent upon the integrity of the growing transcript and is reflected in a significant decrease in the density of the chromatin during transcription. The decrease in density appears to be due to an association between the growing transcript and a large zone of lower density, possibly the nuclear matrix. These interactions are preserved after fixation of the nuclear material with formaldehyde. Hormonal induction of transcriptional activity causes a shift of the genetic material for the stimulated gene from the high density domain to the low density region. The vast majority of the polymerase II which is engaged with the chromosomal material is also found in this lower density zone. We find that most of the fast form of histone acetylation occurs on those histones which are associated with the active chromatin, further supporting the idea that this modification is involved in some way with the transcriptional process. The merits of this approach are discussed, as are the possibilities for its further exploitation.