Eukaryotic RNA Polymerases

Eukaryotes have three RNA polymerases with distinct roles. RNA Polymerase I transcribes most ribosomal RNA genes. RNA Polymerase III transcribes transfer RNA genes, some small nuclear RNA, and other small RNA genes. RNA Polymerase II transcribes the majority of protein-encoding genes. Though structurally similar to each other and to bacterial polymerases, each specializes in different RNA classes.

The carboxy-terminal domain of RNA Polymerase II serves as a binding site for transcription factors that regulate its enzymatic activity. The binding of these factors depends on the phosphorylation pattern of this domain. This phosphorylation-dependent mechanism allows precise control of transcription initiation and elongation in eukaryotes.

All three eukaryotic RNA polymerases require specific transcription factors to function properly. These proteins remain attached to the RNA polymerase to guide the direction of RNA synthesis on the template DNA strand. The TATA-binding protein is common to all three polymerases, ensuring coordinated regulation of different gene classes.

Poly(A) dependent termination combines polyadenylation of the mRNA transcript with RNA Polymerase II termination. While the polymerase continues transcribing past the gene end, the transcript is cleaved at an internal site. The upstream part is released and receives a poly(A) tail, while a 5'-exonuclease digests the downstream product, helping the polymerase dissociate from the DNA template.

Once RNA strand elongation is complete, the RNA polymerase and associated transcription factors must disassemble and release the mRNA transcript. This release is essential for mRNA processing and export from the nucleus. Unlike bacterial polymerases that terminate at specific sequences, eukaryotic polymerases require coordinated cleavage and exonuclease activity to properly terminate and release the transcript.

Eukaryotic RNA polymerases share significant structural and sequence similarities with bacterial RNA polymerase. RNA Polymerase II is most similar to bacterial RNAP in terms of structural organization and folding topologies of enzyme subunits. However, despite these structural similarities, their mechanisms of action differ substantially, particularly in transcription initiation, elongation, and termination processes.

Bacterial genes contain specific DNA sequences that direct RNA polymerase to terminate at precise locations. In contrast, protein-encoding genes transcribed by eukaryotic RNA Polymerase II lack these specific termination sequences. Instead, eukaryotes rely on poly(A) dependent termination, which combines transcript cleavage with polyadenylation and exonuclease digestion to terminate transcription.