Large-scale chromatin decondensation and recondensation regulated by transcription from a natural promoter

We have examined the relationship between transcription and chromatin struc ture using a tandem array of the mouse mammary tumor virus (MMTV) promoter driving a ras reporter. The array was visualized as a distinctive fluoresce nt structure in live cells stably transformed with a green fluorescent prot ein (GFP)-tagged glucocorticoid receptor (GR), which localizes to the repea ted MMTV elements after steroid hormone treatment. Also found at the array by immunofluorescence were two different steroid receptor coactivators (SRC 1 and CBP) with acetyltransferase activity, a chromatin remodeler (BRG1), a nd two transcription factors (NFI and AP-2). Within 3 h after hormone addit ion, arrays visualized by GFP-GR or DNA fluorescent in situ hybridization ( FISH) decondensed to varying degrees, in the most pronounced cases from a s imilar to0.5-mum spot to form a fiber 1-10 mum long. Arrays later recondens ed by 3-8 h of hormone treatment. The degree of decondensation was proporti onal to the amount of transcript produced by the array as detected by RNA F ISH. Decondensation was blocked by two different drugs that inhibit polymer ase II, 5,6-dichloro-1-beta -D-ribofuranosylbenzimidazole (DRB) and alpha - amanitin. These observations demonstrate a role for polymerase in producing and maintaining decondensed chromatin. They also support fiber-packing mod els of higher order structure and suggest that transcription from a natural promoter may occur at much higher DNA-packing densities than reported prev iously.