RNSpolimeráz

RNSpolimeráz (RNA polymerase) denotes a family of enzymes that synthesize RNA from a template. In most cellular systems, it is a DNA-dependent RNA polymerase that transcribes genes from a DNA template into a complementary RNA strand. Bacteria possess a single RNA polymerase that requires a sigma factor for promoter recognition. The bacterial core enzyme is made of multiple subunits and works with sigma to locate promoters, initiate transcription, and then proceed to elongation.

Eukaryotes have three main nuclear RNA polymerases: RNA polymerase I, II, and III, each dedicated to different gene sets. RNA Pol I transcribes ribosomal RNA components, RNA Pol II transcribes mRNA and many noncoding RNAs, and RNA Pol III transcribes transfer RNA and other small RNAs. Mitochondria and chloroplasts have their own RNA polymerases, often derived from bacterial ancestors, reflecting endosymbiotic origins.

Mechanism: The polymerase catalyzes the formation of phosphodiester bonds, incorporating ribonucleotides with the help of divalent metal ions, typically Mg2+. It progresses through initiation, promoter escape, elongation, and termination. In eukaryotes, transcription involves a set of general transcription factors and coactivators that recruit the polymerase to promoters, while bacterial transcription relies on sigma factors for promoter specificity.

Regulation and inhibition: Transcription is regulated by DNA-binding factors, chromatin structure, signaling pathways, and other regulatory inputs. Antibiotics such as rifampicin inhibit bacterial RNA polymerase; toxins like alpha-amanitin inhibit eukaryotic RNA polymerase II, illustrating the central role of RNAPs in gene expression.

RNA-dependent RNA polymerases also exist in some viruses and in RNA silencing pathways, copying RNA templates rather than DNA.