The molecular basis of eucaryotic transcription.

Thanks to the Nobel Foundation for permission to publish this Lecture. We report here the Nobel Lecture delivered by Professor RD Kornberg describing his research in the understanding of transcription in eucaryotes. The amazing work by Professor Kornberg goes from the discovery of the nucleosome to the structural and functional studies of pol II transcription complexes. His research sheds light on fundamental molecular biology problems such as transcription initiation, fidelity of transcription, RNA release at the end of transcription, and many more. This is a beautiful report on how structural and functional studies can be combined to really understand in an accurate and detailed way how proteins combine in huge molecular complexes to regulate one of the most important cellular processes: gene transcription.

Quantitation of the RNA polymerase II transcription machinery in yeast.

TAP tags and dot blot analysis have been used to measure the amounts of RNA polymerase II transcription proteins in crude yeast extracts. The measurements showed comparable amounts of RNA polymerase II, TFIIE, and TFIIF, lower levels of TBP and TFIIB, and still lower levels of Mediator and TFIIH. These findings are consistent with the presumed roles of the transcription proteins, but do not support the idea of their recruitment in a single large complex to RNA polymerase II promoters. The approach employed here can be readily extended to quantitative analysis of the entire yeast proteome.

The eukaryotic gene transcription machinery.

Seven purified proteins may be combined to reconstitute regulated, promoter-dependent RNA polymerase II transcription: five general transcription factors, Mediator, and RNA polymerase II. The entire system has been conserved across species from yeast to humans. The structure of RNA polymerase II, consisting of 10 polypeptides with a mass of about 500 kDa, has been determined at atomic resolution. On the basis of this structure, that of an actively transcribing RNA polymerase II complex has been determined as well.

A multiprotein complex that interacts with RNA polymerase II elongator.

A three-subunit Hap complex that interacts with the RNA polymerase II Elongator was isolated from yeast. Deletions of genes for two Hap subunits, HAP1 and HAP3, confer pGKL killer-insensitive and weak Elongator phenotypes. Preferential interaction of the Hap complex with free rather than RNA polymerase II-associated Elongator suggests a role in the regulation of Elongator activity.

Selenomethionine incorporation in Saccharomyces cerevisiae RNA polymerase II.

A protocol for the incorporation of SeMet into yeast proteins is described. Incorporation at a level of about 50% suffices for the location of Se sites in an anomalous difference Fourier map of the 0.5 MDa yeast RNA polymerase II. This shows the utility of the approach as an aid in the model-building of large protein complexes.

Structural basis of transcription: an RNA polymerase II elongation complex at 3.3 A resolution.

The crystal structure of RNA polymerase II in the act of transcription was determined at 3.3 A resolution. Duplex DNA is seen entering the main cleft of the enzyme and unwinding before the active site. Nine base pairs of DNA-RNA hybrid extend from the active center at nearly right angles to the entering DNA, with the 3' end of the RNA in the nucleotide addition site. The 3' end is positioned above a pore, through which nucleotides may enter and through which RNA may be extruded during back-tracking. The 5'-most residue of the RNA is close to the point of entry to an exit groove. Changes in protein structure between the transcribing complex and free enzyme include closure of a clamp over the DNA and RNA and ordering of a series of "switches" at the base of the clamp to create a binding site complementary to the DNA-RNA hybrid. Protein-nucleic acid contacts help explain DNA and RNA strand separation, the specificity of RNA synthesis, "abortive cycling" during transcription initiation, and RNA and DNA translocation during transcription elongation.

Structural basis of transcription: RNA polymerase II at 2.8 angstrom resolution.

Structures of a 10-subunit yeast RNA polymerase II have been derived from two crystal forms at 2.8 and 3.1 angstrom resolution. Comparison of the structures reveals a division of the polymerase into four mobile modules, including a clamp, shown previously to swing over the active center. In the 2.8 angstrom structure, the clamp is in an open state, allowing entry of straight promoter DNA for the initiation of transcription. Three loops extending from the clamp may play roles in RNA unwinding and DNA rewinding during transcription. A 2.8 angstrom difference Fourier map reveals two metal ions at the active site, one persistently bound and the other possibly exchangeable during RNA synthesis. The results also provide evidence for RNA exit in the vicinity of the carboxyl-terminal repeat domain, coupling synthesis to RNA processing by enzymes bound to this domain.

RSC unravels the nucleosome.

RSC and SWI/SNF chromatin-remodeling complexes were previously reported to generate a stably altered nucleosome. We now describe the formation of hybrids between nucleosomes of different sizes, showing that the stably altered structure is a noncovalent dimer. A basis for dimer formation is suggested by an effect of RSC on the supercoiling of closed, circular arrays of nucleosomes. The effect may be explained by the interaction of RSC with DNA at the ends of the nucleosome, which could lead to the release 60--80 bp or more from the ends. DNA released in this way may be trapped in the stable dimer or lead to alternative fates such as histone octamer transfer to another DNA or sliding along the same DNA molecule.

Structural organization of yeast and mammalian mediator complexes.

Structures of yeast Mediator complex, of a related complex from mouse cells and of thyroid hormone receptor-associated protein complex from human cells have been determined by three-dimensional reconstruction from electron micrographs of single particles. All three complexes show a division in two parts, a "head" domain and a combined "middle-tail" domain. The head domains of the three complexes appear most similar and interact most closely with RNA polymerase II. The middle-tail domains show the greatest structural divergence and, in the case of the tail domain, may not interact with polymerase at all. Consistent with this structural divergence, analysis of a yeast Mediator mutant localizes subunits that are not conserved between yeast and mammalian cells to the tail domain. Biochemically defined Rgr1 and Srb4 modules of yeast Mediator are then assigned to the middle and head domains.

Electron crystal structure of the transcription factor and DNA repair complex, core TFIIH.

Core TFIIH from yeast, made up of five subunits required both for RNA polymerase II transcription and nucleotide excision DNA repair, formed 2D crystals on charged lipid layers. Diffraction from electron micrographs of the crystals in negative stain extended to about 13 angstrom resolution, and 3D reconstruction revealed several discrete densities whose volumes corresponded well with those of individual TFIIH subunits. The structure is based on a ring of three subunits, Tfb1, Tfb2, and Tfb3, to which are appended several functional moieties: Rad3, bridged to Tfb1 by SsI1; SsI2, known to interact with Tfb2; and Kin28, known to interact with Tfb3.

Mediator of transcriptional regulation.

Three lines of evidence have converged on a multiprotein Mediator complex as a conserved interface between gene-specific regulatory proteins and the general transcription apparatus of eukaryotes. Mediator was discovered as an activity required for transcriptional activation in a reconstituted system from yeast. Upon resolution to homogeneity, the activity proved to reside in a 20-protein complex, which could exist in a free state or in a complex with RNA polymerase II, termed holoenzyme. A second line of evidence came from screens in yeast for mutations affecting transcription. Two-thirds of Mediator subunits are encoded by genes revealed by these screens. Five of the genetically defined subunits, termed Srbs, were characterized as interacting with the C-terminal domain of RNA polymerase II in vivo, and were shown to bind polymerase in vitro. A third line of evidence has come recently from studies in mammalian transcription systems. Mammalian counterparts of yeast Mediator were shown to interact with transcriptional activator proteins and to play an essential role in transcriptional regulation. Mediator evidently integrates and transduces positive and negative regulatory information from enhancers and operators to promoters. It functions directly through RNA polymerase II, modulating its activity in promoter-dependent transcription. Details of the Mediator mechanism remain obscure. Additional outstanding questions include the patterns of promoter-specificity of the various Mediator subunits, the possible cell-type-specificity of Mediator subunit composition, and the full structures of both free Mediator and RNA polymerase II holoenzyme.

Mediator-nucleosome interaction.

Mediator, a multiprotein complex involved in the regulation of RNA polymerase II transcription, binds to nucleosomes and acetylates histones. Three lines of evidence identify the Nut1 subunit of Mediator as responsible for the histone acetyltransferase (HAT) activity. An "in-gel" HAT assay reveals a single band of the appropriate size. Sequence alignment shows significant similarity of Nut1 to the GCN5-related N-acetyltransferase superfamily. Finally, recombinant Nut1 exhibits HAT activity in an in-gel assay.

Defocus-gradient corrected back-projection.

Three-dimensional reconstructions of icosahedral viruses from cryoelectron microscope images have reached resolutions where the microscope depth of field is a significant resolution-limiting factor. An analytical treatment presented here shows how the depth of field limitation can be understood as an envelope function which gradually attenuates the signal, starting well before the numerical depth of field is actually reached. A simple modification to the well-known back-projection reconstruction algorithm is described, called the defocus-gradient corrected back-projection, which computationally corrects for the contrast transfer function along a defocus gradient. Computer simulations demonstrate how the algorithm effectively eliminates the depth of field limitation.

Architecture of RNA polymerase II and implications for the transcription mechanism.

A backbone model of a 10-subunit yeast RNA polymerase II has been derived from x-ray diffraction data extending to 3 angstroms resolution. All 10 subunits exhibit a high degree of identity with the corresponding human proteins, and 9 of the 10 subunits are conserved among the three eukaryotic RNA polymerases I, II, and III. Notable features of the model include a pair of jaws, formed by subunits Rpb1, Rpb5, and Rpb9, that appear to grip DNA downstream of the active center. A clamp on the DNA nearer the active center, formed by Rpb1, Rpb2, and Rpb6, may be locked in the closed position by RNA, accounting for the great stability of transcribing complexes. A pore in the protein complex beneath the active center may allow entry of substrates for polymerization and exit of the transcript during proofreading and passage through pause sites in the DNA.

Subunit interactions in yeast transcription/repair factor TFIIH. Requirement for Tfb3 subunit in nucleotide excision repair.

A yeast strain harboring a temperature-sensitive allele of TFB3 (tfb3(ts)), the 38-kDa subunit of the RNA polymerase II transcription/nucleotide excision repair factor TFIIH, was found to be sensitive to ultraviolet (UV) radiation and defective for nucleotide excision repair in vitro. Interestingly, tfb3(ts) failed to grow on medium containing caffeine. A comprehensive pairwise two-hybrid analysis between yeast TFIIH subunits identified novel interactions between Rad3 and Tfb3, Tfb4 and Ssl1, as well as Ssl2 and Tfb2. These interactions have facilitated a more complete model of the structure of TFIIH and the nucleotide excision repairosome.

Eukaryotic transcriptional control.

Some 30 years ago, following the elucidation of transcriptional control in prokaryotes, attention turned to the corresponding problem in eukaryotes: how are so many genes transcribed in a cell-type-specific, developmentally regulated manner? The answer has been found in two modes of regulation, one involving chromatin and the other the chief transcribing enzyme, RNA polymerase II. Although basic features of the prokaryotic mechanism have been preserved, the demands of eukaryotic transcription control are met by a huge increase in complexity and by the addition of new layers to the transcription apparatus. Discovering the components of this apparatus has been a major theme of research over the past three decades; unravelling the mechanisms is a challenge for the future.

Electron crystal structure of an RNA polymerase II transcription elongation complex.

The structure of an actively transcribing complex, containing yeast RNA polymerase II with associated template DNA and product RNA, was determined by electron crystallography. Nucleic acid, in all likelihood the "transcription bubble" at the active center of the enzyme, occupies a previously noted 25 A channel in the protein structure. Details are indicative of a roughly 90 degrees bend of the DNA between upstream and downstream regions. The DNA apparently lies entirely on one face of the polymerase, rather than passing through a hole to the opposite side, as previously suggested.

Yeast RNA polymerase II at 5 A resolution.

Appropriate treatment of X-ray diffraction from an unoriented 18-heavy atom cluster derivative of a yeast RNA polymerase II crystal gave significant phase information to 5 A resolution. The validity of the phases was shown by close similarity of a 6 A electron density map to a 16 A molecular envelope of the polymerase from electron crystallography. Comparison of the 6 A X-ray map with results of electron crystallography of a paused transcription elongation complex suggests functional roles for two mobile protein domains: the tip of a flexible arm forms a downstream DNA clamp; and a hinged domain may serve as an RNA clamp, enclosing the transcript from about 8-18 residues upstream of the 3'-end in a tunnel.

Chromatin-modifying and -remodeling complexes.

Nucleosomes have long been known to inhibit DNA transactions on chromosomes and a remarkable abundance of multiprotein complexes that either enhance or relieve this inhibition have been described. Most is known about chromatin-remodeling complexes that perturb nucleosome structure.