RNA polymerase II

RNA polymerase II is the principal nuclear polymerase for protein-coding genes in eukaryotes. It reads one DNA strand and synthesizes a complementary RNA strand, usually beginning with a promoter-directed initiation event and yielding a pre-mRNA that later undergoes processing.¹

The enzyme is a roughly 12-subunit complex whose largest subunit, RPB1, forms much of the catalytic cleft and contains the C-terminal domain (CTD). Structural studies show a central cleft, a DNA-RNA hybrid region, and a clamp that helps hold the transcription bubble in place.¹ The active center uses a metal-dependent mechanism related to that of other multisubunit RNA polymerases. In mammals, the gene encoding RPB1 is POLR2A.⁵

Transcription begins when general transcription factors assemble RNA polymerase II at a promoter, often with help from the Mediator complex and gene-specific regulatory proteins. TFIIH contributes both promoter-opening activity and phosphorylation of the RPB1 CTD, helping the polymerase escape the promoter and enter productive elongation.²

During elongation, the polymerase advances along DNA while maintaining a short RNA-DNA hybrid and repeatedly adding ribonucleotides to the RNA 3′ end. Nucleosomes and chromatin-associated factors can impede or regulate this movement. Termination is coupled to cleavage and polyadenylation of many pre-mRNAs; after the downstream RNA is released, polymerase II is recycled for another transcription cycle. The precise transition between initiation, pausing, elongation, and termination is regulated rather than purely mechanical.

The RPB1 C-terminal domain coordinates transcription with RNA processing by changing its phosphorylation pattern as polymerase II moves through a gene. In humans, the CTD contains 52 tandem repeats of the consensus heptapeptide YSPTSPS; phosphorylation of serine residues creates binding sites for stage-specific regulatory proteins.³

Early in transcription, CTD phosphorylation helps recruit enzymes that install the 5′ cap on nascent RNA. Other CTD states are associated with RNA splicing, 3′-end formation, and termination, allowing these events to occur while the transcript is still being synthesized.² The CTD is therefore more than a structural tail: it acts as a dynamic interaction platform whose modification cycle helps organize gene expression in time and space.

RNA polymerase II also transcribes many RNAs that do not encode proteins, including several classes of regulatory and small nuclear RNAs. Its transcriptional output is consequently broader than the protein-coding genome alone, although the exact division of labor among polymerases varies among organisms.⁶

A notable vulnerability is that the polymerase can become trapped by DNA lesions or by persistent RNA-DNA structures known as R-loops, which can threaten genome stability. Polymerase II is also a major target of the α-amanitin toxin from certain Amanita mushrooms; the toxin inhibits the enzyme by binding within its elongation machinery.¹ Structural work on Saccharomyces cerevisiae polymerase II provided an early high-resolution view of the conserved transcription apparatus and remains an important reference for interpreting eukaryotic mechanisms.⁴

Pre-mRNA

The initial RNA transcript made from a protein-coding gene before capping, splicing, and 3′-end processing are complete.

C-terminal domain

The repetitive tail of RPB1 that recruits and regulates factors involved in transcription and RNA processing.

Mediator

A multisubunit coactivator complex that connects gene-specific regulatory proteins with the general transcription machinery.

TFIIH

A general transcription factor complex involved in promoter opening, CTD phosphorylation, and nucleotide-excision repair.

R-loop

A three-stranded structure containing an RNA-DNA hybrid and a displaced single DNA strand.