Mechanisms for diversity in gene expression patterns.

Despite the relatively low number of transcriptional regulatory proteins, the number of possible combinations that act in particular cell types at specific times and in response to appropriate extracellular stimuli is enormous. In considering the regulatory patterns of a particular gene, the critical determinants of diversity are the specific promoter sequences that govern the potential DNA-binding proteins which function either directly or indirectly in association with other proteins; constellations of proteins in the nucleus and their transcriptional activities; and synergistic or antagonistic protein-protein interactions. Although some of these regulatory principles operate in prokaryotes, the combinatorial nature of the transcriptional activation process, the existence of multiprotein families, and the prevalence of heteromeric protein complexes are characteristic of eukaryotic cells and are essential for the extraordinary complexity of gene expression patterns in multicellular organisms.