Visualization of gene activity in living cells

Chromatin structure is thought to play a critical role in gene expression. Using the lac operator/repressor system and two colour variants of green fl uorescent protein (GFP), we developed a system to visualize a gene and its protein product directly in living cells, allowing us to examine the spatia l organization and timing of gene expression in vivo. Dynamic morphological changes in chromatin structure, from a condensed to an open structure, wer e observed upon gene activation, and targeting of the gene product, cyan fl uorescent protein (CFP) reporter to peroxisomes was visualized directly in living cells. We found that the integrated gene locus was surrounded by a p romyelocytic leukaemia (PML) nuclear body. The association was transcriptio n independent but was dependent upon the direct in vivo binding of specific proteins (EYFP/lac repressor, tetracycline receptor/VP16 transactivator) t o the locus. The ability to visualize gene expression directly in living ce lls provides a powerful system with which to study the dynamics of nuclear events such as transcription, RNA processing and DNA repair.