transcriptionpolymerase

Transcription polymerase, commonly referred to as RNA polymerase, is the enzyme responsible for transcription—the synthesis of RNA using a DNA template. It catalyzes the formation of phosphodiester bonds from ribonucleoside triphosphates and proceeds in the 5' to 3' direction, reading the DNA template in the 3' to 5' direction. It does not require a primer, unlike DNA polymerases.

In bacteria, a single multisubunit RNA polymerase conducts transcription for all RNA classes, aided by a sigma factor that directs promoter recognition. In archaea and eukaryotes, transcription is more specialized: RNA polymerase I transcribes most ribosomal RNA, RNA polymerase II transcribes mRNA and many small RNAs, and RNA polymerase III transcribes transfer RNA and other small RNAs. Eukaryotic transcription relies on numerous transcription factors and promoter elements, and it is frequently modulated by chromatin structure and epigenetic marks.

The transcription cycle starts with promoter recognition and open complex formation, followed by initiation and promoter clearance, elongation, and termination. The polymerase moves along the DNA, synthesizing a complementary RNA strand that exits the enzyme. In bacteria, the core enzyme and sigma factor together determine promoter specificity; in eukaryotes, a suite of general and gene-specific factors coordinate initiation. In many eukaryotes, transcription is coupled with RNA processing, including capping, splicing, and polyadenylation, which occur co-transcriptionally.

Viruses also rely on transcription polymerases. Some contain viral RNA-dependent RNA polymerases that transcribe from RNA templates, while others encode specialized transcription enzymes. Transcription polymerases are essential for gene expression and are important targets in medicine (for example, antibiotics such as rifampicin inhibit bacterial RNA polymerase) and biotechnology (in vitro transcription uses phage-derived RNA polymerases like T7 RNA polymerase).