RNA Polymerase

RNA polymerase is an enzyme that synthesizes ribonucleic acid (RNA) by joining ribonucleotides in an order determined by a template. In cellular biology, the term usually refers to DNA-dependent RNA polymerase, which copies information from DNA into RNA during transcription. This activity produces both protein-coding transcripts and functional noncoding RNAs. RNA polymerases occur in several structurally distinct families; the principal cellular transcription enzymes are large, multisubunit complexes. (sciencedirect.com)

Catalytic mechanism

RNA polymerase extends RNA in the 5′ → 3′ direction, adding each new nucleotide to the existing chain’s 3′ end. For a DNA-dependent enzyme, the template strand is read in the opposite, 3′ → 5′ direction. The substrates are the four ribonucleoside triphosphates—ATP, GTP, CTP, and UTP—which supply both the building blocks and the chemical energy for synthesis. (pubmed.ncbi.nlm.nih.gov)

During nucleotide addition, the RNA’s terminal 3′ hydroxyl group attacks the incoming nucleotide’s alpha phosphate. A new phosphodiester bond forms, and pyrophosphate is released. Catalytic magnesium ions help position the substrates and support the reaction. Structural studies of both multisubunit and single-subunit polymerases support a two-metal-ion catalytic mechanism. (pubmed.ncbi.nlm.nih.gov)

The active site selects nucleotides through interactions involving the template base, the substrate, and surrounding protein elements. Correct base pairing is therefore essential but is not the only determinant of specificity: conformational changes also help distinguish suitable substrates from incorrect nucleotides and deoxyribonucleotides. (nature.com)

Transcriptional RNA polymerases can initiate a chain de novo, without a pre-existing RNA primer. Initiation nevertheless differs from established elongation: early complexes often release short, abortive transcripts before making the transition to productive RNA synthesis. (nature.com)

Major cellular types

Bacterial RNA polymerase

Bacteria generally use one principal multisubunit RNA polymerase to transcribe their genomic DNA. Its core contains two alpha subunits and one each of the beta, beta-prime, and omega subunits, conventionally written α₂ββ′ω. The large beta and beta-prime subunits form much of the catalytic machinery. (sciencedirect.com)

For promoter-specific initiation, the core associates with a sigma factor. Sigma recognizes features of a promoter and helps establish the transcription start site. The resulting complex is called the holoenzyme. After initial RNA synthesis, successful promoter escape produces an elongation complex capable of synthesizing a much longer transcript. (nature.com)

Archaeal RNA polymerase

Archaea also generally possess one principal multisubunit transcription polymerase, but their initiation machinery resembles the eukaryotic system rather than the bacterial sigma-factor system. Basal initiation uses TATA-binding protein and transcription factor B, which help recruit the polymerase and establish start-site-specific transcription. (sciencedirect.com)

Eukaryotic nuclear RNA polymerases

Eukaryotes possess three major nuclear RNA polymerases with different transcriptional responsibilities:

Polymerase	Principal products
RNA polymerase I	The large precursor of most [[ribosomal-rna
RNA polymerase II	Precursors of [[messenger-rna
RNA polymerase III	[[transfer-rna

The three enzymes share related structural components but differ in subunit composition and transcriptional specialization. In particular, 5S rRNA is made separately by polymerase III rather than as part of the large polymerase-I precursor. (sciencedirect.com)

Plants have two additional nuclear enzymes, RNA polymerases IV and V. These are specialized relatives of polymerase II involved in RNA-mediated gene silencing and DNA methylation. Their roles are distinct: polymerase IV participates in the production of small interfering RNAs, whereas polymerase V generates noncoding transcripts at target regions that help recruit silencing machinery. (sciencedirect.com)

Organellar and bacteriophage polymerases

Not all DNA-dependent RNA polymerases have the large multisubunit architecture. Human mitochondrial RNA polymerase belongs to a single-subunit family related to enzymes found in certain bacteriophages. These enzymes have a different overall structure while retaining template-directed nucleotide selection and metal-dependent catalysis. (nature.com)

A prominent example is T7 RNA polymerase, encoded by bacteriophage T7. Its promoter specificity and ability to produce long RNA transcripts make it an important experimental and industrial enzyme. (rcsb.org)

Structure and transcriptional dynamics

Multisubunit polymerases contain a deep cleft that accommodates the nucleic acids. A movable clamp helps control their engagement with the enzyme, while channels and surface grooves guide the template and emerging RNA. Structures of yeast polymerase II revealed several mobile modules, demonstrating that transcription requires coordinated conformational changes rather than a rigid catalytic assembly. (pubmed.ncbi.nlm.nih.gov)

Elongation is not uniformly continuous. Polymerases can pause and sometimes move backward along the template, a process called backtracking. This displaces the RNA’s 3′ end from the active site and temporarily prevents further nucleotide addition. Productive elongation can resume after forward movement or cleavage of the displaced RNA segment. (pubmed.ncbi.nlm.nih.gov)

Backtracking and transcript cleavage also contribute to proofreading. An incorrectly incorporated nucleotide can promote a backtracked state, allowing the recently synthesized RNA segment to be removed. In bacterial systems, accessory proteins such as GreA and GreB stimulate cleavage and shorten some backtracking-associated pauses. Not all backtracking is caused by errors; template and RNA sequences can also induce it. (pubmed.ncbi.nlm.nih.gov)

Polymerase II and RNA processing

RNA polymerase II connects RNA synthesis with RNA maturation. Its largest subunit contains a flexible carboxy-terminal domain, or CTD, consisting of repeated seven-amino-acid motifs with the consensus sequence Tyr–Ser–Pro–Thr–Ser–Pro–Ser. The domain acts as an interaction platform for proteins involved in transcription and RNA processing. (nature.com)

Changes in CTD phosphorylation help regulate these interactions. Associated factors participate in 5′ capping, splicing, and 3′-end processing and polyadenylation. These reactions are carried out by recruited enzymes rather than by the polymerase’s RNA-synthesis active site. Many processing events are coupled to transcription and can begin before the transcript is complete. (nature.com)

RNA-dependent RNA polymerases

In a broader usage, RNA polymerase also includes RNA-dependent RNA polymerase, abbreviated RdRP, which synthesizes RNA using another RNA molecule as its template. Many RNA viruses use these enzymes to replicate their genomes and produce viral transcripts. Their template requirement distinguishes them from the DNA-dependent enzymes responsible for ordinary cellular transcription. (pdb101.rcsb.org)

Viral polymerases can operate within larger complexes. For example, the Ebola virus polymerase associates with VP35, and structural studies have identified different conformations associated with RNA synthesis. Thus, the term “RNA polymerase” describes a catalytic activity shared by enzymes with substantially different architectures and biological roles. (nature.com)

Research applications and inhibition

T7 RNA polymerase is widely used for in vitro transcription, in which a DNA template is converted into RNA outside a cell. Applications include preparing experimental RNA and manufacturing messenger RNA. The process can also generate unwanted products, including short abortive transcripts and double-stranded RNA. Enzyme engineering has therefore sought to improve product purity as well as yield. (nature.com)

RNA polymerases are also targets of inhibitory compounds. Rifampicin binds bacterial polymerase and interferes with early productive transcription. Changes in its binding site, encoded within the rpoB gene for the beta subunit, can alter drug binding and confer resistance. These interactions provide experimentally tractable examples of how polymerase structure affects inhibitor sensitivity. (nature.com)

Historical development

In 1969, Robert G. Roeder and William J. Rutter reported distinct DNA-dependent RNA polymerase activities in eukaryotic organisms. Their isolation of three activities from developing sea urchin embryos helped establish the distinction among nuclear polymerases I, II, and III. (nature.com)

High-resolution X-ray crystallography subsequently revealed the architecture of polymerase II and its catalytic center. A 2001 study resolved yeast polymerase-II structures at 2.8 and 3.1 ångström resolution, exposing mobile elements and active-site metal ions. Roger D. Kornberg received the 2006 Nobel Prize in Chemistry for studies of the molecular basis of eukaryotic transcription. (pubmed.ncbi.nlm.nih.gov)