Imaging RNA Polymerase II transcription sites in living cells

Over the past twenty years, exciting developments in optical and molecular imaging approaches have allowed researchers to examine with unprecedented resolution the spatial organization of transcription sites in the nucleus. An attractive model that has developed from these studies is that active genes cluster to preformed transcription factories that contain multiple active RNA polymerases and transcription factor proteins required for efficient mRNA biogenesis. However, this model has been extensively debated in part due to the fact transcription factories and their features have only been documented in fixed cells. In this review, we will focus on recent live-cell imaging studies that are changing our understanding of transcription factories.