RNA polymerase

If transcription is copying a recipe by hand, RNA polymerase is the hand — the molecular machine that does the actual writing. It is a large enzyme that clamps onto DNA, pries the two strands apart, reads one of them, and stitches together a matching RNA chain one building block at a time. Nothing in transcription happens without it.

Mechanically, RNA polymerase is a multi-protein complex shaped a bit like a crab's claw that grips the DNA. Inside, it holds a short open bubble of unpaired DNA and a catalytic site where it joins incoming ribonucleotides. Each new nucleotide is added to the 3' end of the growing RNA, and the chemistry is driven by the same kind of energy-rich building blocks used elsewhere in the cell — ribonucleoside triphosphates (ATP, GTP, CTP, UTP) that snap on and release two phosphates as the bond forms. Crucially, unlike the DNA polymerase of replication, RNA polymerase can start a brand-new chain from scratch — it needs no pre-made primer. It also has only modest proofreading, so transcription is a bit less accurate than replication, which is acceptable because RNA copies are temporary and made in bulk.

Different life forms run different numbers of these machines. Bacteria get by with a single core RNA polymerase that, guided by a swappable sigma factor, transcribes all their genes. Eukaryotes split the labour among three specialized enzymes — RNA polymerases I, II, and III — each making a different class of RNA. Because RNA polymerase is essential and bacterial versions differ from ours, it is a prime drug target: the antibiotic rifampicin, used against tuberculosis, works by jamming the bacterial enzyme.