Eukaryotic and Prokaryotic RNA Polymerases: Subunit Composition

RNA polymerases are complex enzymes responsible for transcription, the process of synthesizing RNA from a DNA template. Their subunit composition varies significantly between prokaryotes and eukaryotes, impacting their function and regulation.

Prokaryotic RNA Polymerase

Bacterial RNA polymerase is a relatively simple enzyme, consisting of a core enzyme and a sigma factor.

Core Enzyme

The core enzyme is composed of five subunits: two α (alpha), one β (beta), one β' (beta prime), and one ω (omega). The α subunits are involved in enzyme assembly and interaction with regulatory proteins. The β and β' subunits form the catalytic core, responsible for RNA synthesis. The ω subunit plays a role in enzyme assembly and stability.

Sigma Factor

The sigma factor is a regulatory subunit that is not permanently associated with the core enzyme. It plays a crucial role in promoter recognition and initiation of transcription. Different sigma factors can recognize different promoter sequences, allowing the cell to regulate gene expression in response to environmental changes.

Eukaryotic RNA Polymerases

Eukaryotes possess multiple RNA polymerases, each responsible for transcribing different classes of RNA.

RNA Polymerase I

Primarily transcribes ribosomal RNA (rRNA) genes.

RNA Polymerase II

Transcribes protein-coding genes, producing messenger RNA (mRNA) precursors. This enzyme is considerably more complex than the bacterial RNA polymerase and consists of numerous subunits, typically more than 12. The exact number and the precise role of each subunit can vary slightly depending on the organism.

RNA Polymerase III

Transcribes transfer RNA (tRNA) genes, 5S rRNA genes, and other small RNA genes. It's also a multi-subunit enzyme.

Subunit Interactions and Function

The precise arrangement and interactions between the subunits are critical for the catalytic activity and regulatory functions of RNA polymerases. Many subunits contribute to DNA binding, unwinding the DNA double helix, nucleotide selection, RNA chain elongation, and termination of transcription.

Further Research

Detailed information on the specific subunit composition and structure of individual RNA polymerases can be found in specialized biochemical and molecular biology literature for specific organisms.