Figure 4. Gene transcription at low and high PolII densities.

During transcription at a moderate or low level the histone octamer is not likely to be displaced from DNA even transiently. However transcription by PolII is accompanied by frequent transient displacement of H2A/H2B dimer(s); this results in extensive and fast transcription-dependent exchange of H2A/H2B histones and possibly in rare exchange of H3/H4 histones. Transcription through nucleosomes and displacement of the dimers could be accomplished by PolII itself; however the rate of elongation is strongly increased by TFIIS and FACT. Re-binding of displaced H2A/H2B dimer to DNA occurs almost immediately after PolII passage and most likely is facilitated by FACT. At a higher density of PolII molecules complete histone octamer can be displaced (by one of the mechanisms described in Fig. 3). Since efficiency of this process depends only on the density of PolII molecules, PolII is the primary player in the reaction. If the rate of transcription is decreased, the octamer rebinds to DNA almost immediately; the recovery of chromatin structure depends upon activity of Spt2, Spt6 FACT and Hir proteins. Other designations are as in Fig. 1.