Glucocorticoid hormone-induced receptor localization to the chromatin fibers formed an injected DNA in Xenopus oocytes

Oocytes from Xenopus laevis have provided a model system for studying the dynamic changes that occur in chromatin during gene activation. We have reconstituted glucocorticoid receptor (GR) induced transcription fi om the mouse mammary tumor virus (MMTV) promoter by intranuclear injection of an MMTV-driven reporter and cytoplasmic injection of synthetic mRNA, into Xenopus oocytes. Here we investigate the intranuclear distribution of injected DNA, which is assembled into chromatin. We show that this chromatin is organized as an intranuclear fibrous network. Unliganded GR is located in the cytosol and hormone triggers its nuclear translocation and association with the chromatin fibers. Furthermore, we analyze the intranuclear distribution of other factors involved in transcription from the MMTV promoter. Indirect immunofluorescence microscopy on cryostat-sectioned oocytes revealed that BRG1, which is a subunit of the SWI/SNF chromatin remodeling complex, as well as RNA polymerase II and recombinantly expressed Xenopus nuclear factor 1-B, are all associated with the endogenous chromosomes and the chromatin fibers formed on injected DNA. This association does not depend on specific DNA binding sites and appears to be nonspecific. (C) 2001 Academic Press.