A combination of structural approaches yields a complete atomic model of the highly biochemically characterized Escherichia coli RNA polymerase, enabling fuller exploitation of E. coli as a model for understanding transcription.

Yeast Mediator proteins interacting with Med17(Srb4) have been expressed at a high level with the use of recombinant baculoviruses and recovered in homogeneous form as a seven subunit, 223 kDa complex. Electron microscopy and single-particle analysis identify this complex as the Mediator head module. The recombinant head module complements "headless" Mediator for the initiation of transcription in vitro. The module interacts with an RNA polymerase II-TFIIF complex, but not with the polymerase or TFIIF alone. This interaction is lost in the presence of a DNA template and associated RNA transcript, recapitulating the release of Mediator that occurs upon the initiation of transcription. Disruption of the head module in a temperature-sensitive mutant in vivo leads to the release of middle and tail modules from a transcriptionally active promoter. The head module evidently controls Mediator-RNA polymerase II and Mediator-promoter interactions.

Transcription is the process in which genetic information encoded in DNA is translated to messenger RNA, which is then used to direct protein synthesis. Precise regulation of transcription underlies cell development and differentiation, and modulates the response of the cell to external factors. Problems with transcription regulation are responsible for a number of human pathologies, most notably cancer. In eukaryotic cells, the basal transcription machinery comprises RNA polymerase II (RNAPII) and its associated general transcription factors. Regulation requires the intervention of a large number of macromolecular complexes, which alter chromatin structure to modulate access to specific portions of the genome, and convey regulatory information to the basal machinery during the transcription initiation process. Although a great deal remains to be understood about the mechanism of transcription regulation, the Mediator complex (first identified in yeast and now detected in humans and other metazoans) seems to play a key role in transducing regulatory information to the basal machinery.