Müller et al. (page 33) have developed a live-cell system that allows analysis of chromatin condensation during transcription from a natural promoter. Using a tandem array of mouse mammary tumor virus (MMTV) promoters driving ras reporter genes, the authors demonstrate that normal transcription can occur in chromatin that is much more condensed than the transcribed chromatin of previous model systems such as lampbrush chromosomes or polytene puffs. Furthermore, instead of the looping out of DNA observed in these systems, the authors observe a linear unraveling that produces domains of variable packing density.
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. ▪