More than thirty years ago Francois Jacob and Jacques Monod described the first paradigm for differential gene expression. Looking back it is amazing how prescient they were because we now know that, in general, eukaryotic genes also contain regulatory sequences in their DNA called enhancers and promoters. A human cell contains about 100,000 genes, but each specific cell type expresses only about 10,000 genes. The production of a specific collection of proteins by a specific cell type is the result of differential gene expression.
Promoters constitute binding sites for RNA polymerase and the general transcription factors.
RNAP's helper proteins are the so-called general transcription factors. In bacteria, generally one protein binds to a single regulatory sequence in the DNA and turns transcription on or off. In eukaryotes, gene transcription is regulated by enhancer DNA sequences that contain not one but multiple regulatory DNA sequence elements that represent binding sites for multiple transcription factors. So it is not a single factor and binding site, but, rather, the specific combination of DNA sequence elements and regulatory proteins that determines which genes will be transcribed in which cell types, a concept known as combinatorial regulation of gene expression. In many cases, viruses that infect mammalian cells have served as model systems for studies of eukaryotic gene regulation.