Eukaryotic transcription initiation

Transcription is defined as the synthesis of RNA from a DNA template. In bacteria, transcription of all genes is catalysed by a single RNA polymerase, whereas in the eukaryotic nucleus, three different RNA polymerases — Pol I, Pol II and Pol III — carry out the synthesis of different classes of RNA. Pol I and Pol III transcribe a limited number of genes encoding ribosomal RNAs, transfer RNAs and small nuclear RNAs. By contrast, Pol II is responsible for transcription of the thousands of protein-encoding genes, and does so using protein machinery comprising approximately 60 polypeptides with a combined molecular mass of more than 3 million daltons (MDa). Here, we describe the complex organization of these transcription factors and their various functions in directing Pol II to promoter DNA and in responding to the array of physiological and developmental cues that are essential for proper gene expression.