Architecture of TFIIIC and its role in RNA polymerase III pre-initiation complex assembly

Transcribing RNA polymerase III observed by electron cryomicroscopy

Electron cryomicroscopy reconstructions of elongating RNA polymerase (Pol) III at 3.9 Å resolution and of unbound Pol III (apo Pol III) in two distinct conformations at 4.6 Å and 4.7 Å resolution allow the construction of complete atomic models of Pol III and provide new functional insights into the adaption of Pol III to fulfill its specific transcription tasks.

RNA Polymerase III Advances: Structural and tRNA Functional Views

RNA synthesis in eukaryotes is divided among three RNA polymerases (RNAPs). RNAP III transcribes hundreds of tRNA genes and fewer additional short RNA genes. We survey recent work on transcription by RNAP III including an atomic structure, mechanisms of action, interactions with chromatin and retroposons, and a conserved link between its activity and a tRNA modification that enhances mRNA decoding. Other new work suggests important mechanistic connections to oxidative stress, autoimmunity and cancer, embryonic stem cell pluripotency, and tissue-specific developmental effects. We consider that, for some of its complex functions, variation in RNAP III activity levels lead to nonuniform changes in tRNAs that can shift the translation profiles of key codon-biased mRNAs with resultant phenotypes or disease states.

Touch, act and go: landing and operating on nucleosomes

Chromatin-associated enzymes are responsible for the installation, removal and reading of precise post-translation modifications on DNA and histone proteins. They are specifically recruited to the target gene by associated factors, and as a result of their activity, they contribute in modulating cell identity and differentiation. Structural and biophysical approaches are broadening our knowledge on these processes, demonstrating that DNA, histone tails and histone surfaces can each function as distinct yet functionally interconnected anchoring points promoting nucleosome binding and modification. The mechanisms underlying nucleosome recognition have been described for many histone modifiers and related readers. Here, we review the recent literature on the structural organization of these nucleosome-associated proteins, the binding properties that drive nucleosome modification and the methodological advances in their analysis. The overarching conclusion is that besides acting on the same substrate (the nucleosome), each system functions through characteristic modes of action, which bring about specific biological functions in gene expression regulation.

A high density of cis-information terminates RNA Polymerase III on a 2-rail track

Transcription termination delineates the 3' ends of transcripts, prevents otherwise runaway RNA polymerase (RNAP) from intruding into downstream genes and regulatory elements, and enables release of the RNAP for recycling. While other eukaryotic RNAPs require complex cis-signals and/or accessory factors to achieve these activities, RNAP III does so autonomously with high efficiency and precision at a simple oligo(dT) stretch of 5-6 bp. A basis for this high density cis-information is that both template and nontemplate strands of the RNAP III terminator carry distinct signals for different stages of termination. High-density cis-information is a feature of the RNAP III system that is also reflected by dual functionalities of the tRNA promoters as both DNA and RNA elements. We review emerging developments in RNAP III termination and single strand nontemplate DNA use by other RNAPs. Use of nontemplate signals by RNAPs and associated transcription factors may be prevalent in gene regulation.