RNAP

RNA polymerase (RNAP) is the enzyme responsible for transcription, copying genetic information from DNA into RNA. It catalyzes the polymerization of ribonucleoside triphosphates into a growing RNA strand using a DNA template. Across life, RNAPs exist in several forms that reflect different transcription programs: bacteria have a single, multifunctional enzyme; eukaryotes possess multiple RNA polymerases (Pol I, II, and III) that transcribe distinct RNA classes; organelles such as mitochondria and chloroplasts carry their own RNA polymerases, which range from single-subunit enzymes to multi-subunit bacterial-like complexes.

In bacteria, the core enzyme comprises beta, beta', alpha2, and omega subunits; association with a sigma factor yields the holoenzyme that recognizes promoter sequences and initiates RNA synthesis. The enzyme forms an open complex by melting promoter DNA, then transitions to elongation after promoter clearance. Termination occurs via rho-dependent or intrinsic mechanisms.

In eukaryotes, Pol II transcribes mRNA and many noncoding RNAs; Pol I transcribes most rRNA, and Pol III transcribes tRNA and other small RNAs. Each polymerase relies on a set of transcription factors and a chromatin context to locate promoters. Pol II features a carboxy-terminal domain that coordinates capping, splicing, and polyadenylation. Termination and RNAP recycling differ among polymerases and involve distinct factors.

RNAPs are essential for gene expression and are targets for drugs and toxins; rifampicin inhibits bacterial RNAP initiation, while alpha-amanitin inhibits eukaryotic Pol II. Understanding RNAP structure and regulation informs studies of transcription, gene regulation, and evolution.