Three-dimensional distribution of DNase I-sensitive chromatin regions in interphase nuclei of embryonal carcinoma cells.

In situ nick-translation allows the visualization of nuclease-sensitive chromatin regions in interphase nuclei. We have analyzed the three-dimensional (3-D) distribution of DNase I-sensitive regions of chromatin in nuclei from mouse P19 embryonal carcinoma cells by making optical sections using confocal scanning laser microscopy. In undifferentiated as well as embryonal carcinoma cells differentiated in vitro, DNase I-sensitive regions of chromatin are observed as discrete spots in the nucleus. These spots represent clusters of DNase I-sensitive sites. By optical sectioning, we show that these spots are preferentially, but not exclusively, localized at the nuclear periphery. No differences were observed in the spatial distribution of DNase I-sensitive sites in P19 EC cells or the differentiated P19 END-2 cells. Furthermore, we did not observe differences in the distribution of DNase I-sensitive chromatin regions during the cell cycle. These findings indicate, at least for P19 mouse embryonal carcinoma cells and their differentiated derivative END-2, that the compartmentalization of DNase I-sensitive chromatin regions is a general characteristic of the nucleus, independent of cell cycle stage or differentiation state. Since evidence has been presented that DNase I-sensitive sites are associated with actively transcribed chromatin, our results indicate that active transcribing chromatin is compartmentalized, preferentially in the periphery of the nucleus.