The amazing complexity of transcription factories.

Multicellular eukaryotes orchestrate immensely complex patterns of gene expression in order to define numerous specialised cell types. Patterns of expression are regulated at the level of DNA modification, chromatin structure, genome architecture and nuclear organisation. Each aspect of these features of regulation is controlled by complex nuclear systems, and each of the systems must interact to define a coherent regulatory network. In proliferating cells, and particularly during development, patterns of gene expression must be preserved in cells that have active DNA repair mechanisms and efficiently duplicate both DNA and the associated chromatin status--the histone code. The systems that regulate DNA synthesis and genome stability are integrated with those that regulate the efficacy of RNA synthesis, such that patterns of gene expression are maintained. This review analyses how different experimental approaches are revealing the amazing complexity of the transcriptionally active compartment of mammalian cells. The functional implications of this complexity will be discussed.