RNApolymerase

RNA polymerase is the enzyme that synthesizes RNA from a DNA template during transcription. It catalyzes the addition of ribonucleoside triphosphates to the 3' end of a growing RNA strand, using energy from NTP hydrolysis. Synthesis proceeds in the 5' to 3' direction and generally does not require a primer.

In bacteria, a single RNA polymerase holoenzyme carries out transcription. The core enzyme consists of two alpha subunits, one beta, and one beta' subunit, with a separate sigma factor that guides promoter recognition and transcription initiation. Promoters typically contain -35 and -10 elements relative to the transcription start site and influence initiation efficiency. After initiation and promoter escape, the enzyme enters elongation. Termination can be rho-dependent, requiring the Rho helicase, or intrinsic, mediated by a GC-rich hairpin followed by a poly-U tract.

In archaea and eukaryotes, transcription relies on more complex polymerases. Eukaryotes have three main nuclear RNA polymerases: Pol I, II, and III, which transcribe different gene sets (rRNA, mRNA and some snRNA/tRNA genes, respectively). Pol II, responsible for mRNA and many noncoding RNAs, requires a suite of general transcription factors to assemble a preinitiation complex at promoter regions, such as TATA-containing promoters. Eukaryotic transcription is closely linked to RNA processing, including 5' capping, splicing, and 3' end polyadenylation. Termination mechanisms vary by polymerase and organism.

Organellar RNA polymerases in mitochondria and chloroplasts resemble bacterial enzymes and often retain sigma-like features for initiation. While many RNA polymerases lack intrinsic proofreading exonuclease activity, fidelity is supported by transcript cleavage factors (e.g., GreA/GreB in bacteria, TFIIS in eukaryotes) and by regulated pausing and backtracking during elongation.