Length of transcription arrays increases over time (in minutes) after drug addition.
In response to steroid hormone treatment, the MMTV promoter is first activated by the glucocorticoid receptor, and then down-regulated over a six-hour period. Expression of GFP-tagged glucocorticoid receptor allows visualization of the MMTV array in live cells. Müller et al. found that chromatin decondensation and recondensenation paralleled the transcriptional activity of the promoter, and that both the establishment and maintenance of the decondensed state required transcription by DNA polymerase II activity.
Transcription in this system can occur at high chromatin packing densities, a phenomenon previously seen only with artificial promoters that have high densities of transcription-factor binding sites. It remains unclear how the transcriptional machinery interacts with the densely packed chromatin, but these questions should be easier to address now that a live-cell system is available. ▪