Fragmentation of coding region chromatin of trp-dioxygenase (to) and tyr-aminotransferase (tat) genes in their active and repressed states by micrococcal nuclease
The packaging of the eukaryotic genome into chromatin severely restricts the access of the transcriptional machinery to the DNA. Recent studies reveal that histones are removed and replaced to enable or restrict, respectively, access of the transcription machinery to regulate transcription. Chromatin disassembly at promoters enables transcriptional activation, whereas promoter chromatin reassembly represses transcription. Histone loss also occurs within transcription units to enable passage of the RNA polymerase, but in this case the histones are rapidly replaced, sometimes by 'variant' histones with specific properties that might serve as a memory of transcriptional competence. Furthermore, the ultimate goal of some epigenetic modifications might well turn out to be the regulation of histone occupancy on the DNA.