Large-scale chromatin organization and the localization of proteins involved in gene expression in human cells

Compartmentalization of the interphase nucleus is an important element in the regulation of gene expression. Here we investigated the functional organization of the interphase nucleus of HeLa cells and primary human fibroblasts. The spatial distribution of proteins involved in transcription (TFIIH and RNA polymerase II) and RNA processing and packaging (hnRNP-U) were analyzed in relation to chromosome territories and large-scale chromatin organization. We present evidence that these proteins are present predominantly in the interchromatin space, inside and between chromosome territories, and are largely excluded by domains of condensed chromatin. We show that they are present throughout the active and inactive X-chromosome territories in primary female fibroblasts, indicating that these proteins can freely diffuse throughout the interchromatin compartment in the interphase nucleus. Furthermore, we established that the in vivo spatial distribution of condensed chromatin in the interphase nucleus does not depend on ongoing transcription. Our data support a conceptually simple model for the functional organization of interphase nuclei.