Figure 9.

Transcriptional activity is confined to the interior compartment. Transcriptional activity within distinct genome compartments was investigated by BrUTP pulse labeling (10 min) of cells replication-labeled on the previous day. BrUTP was detected with TRITC fluorescence, whereas replication labeling was performed with FITC-dUTP. HeLa cells (a–c) and CHO cells (d–f) were investigated. Identical nuclear planes were imaged with regard to FITC detection (b and e, depicted in green) and TRITC detection (c and f, depicted in red). Merges of corresponding FITC and TRITC signals are depicted in a and d (colocalizing signals appear yellow). Transcriptional activity, indicated by BrUTP incorporation into nascent RNA (c and f, and, a and d, red) is confined to the interior compartment (a and b, green, colocalizing with the type I pattern). The TRITC signal in the upper left regions of a and c is due to an adjacent BrUTP-labeled nucleus that is not replication-labeled. d–f depict the exclusion of RNA synthesis from the peripheral compartments (d and e, green). Arrows in f show examples of perinuclear and perinucleolar regions strongly labeled by the type III replication labeling pattern (d and e, green) that display no TRITC signal (no RNA synthesis).