RNA polymerase
/ AR-en-AY puh-LIM-er-ase /
Picture a photocopier that does not just scan a page but physically walks along the original, line by line, spitting out the copy as it goes. RNA polymerase is the cell's molecular version of that machine. It is the enzyme — a large protein — that actually carries out transcription, traveling along a gene and building the RNA copy one letter at a time.
In detail, RNA polymerase does several jobs at once. It pries the two DNA strands apart to expose the letters, it reads the exposed template strand, it grabs the matching free RNA building blocks (ribonucleotides) floating nearby, and it links them into a growing chain, always adding to the 3-prime end. It moves in steps, releasing two phosphate groups as fuel each time it adds a letter. Unlike DNA polymerase, RNA polymerase can start a brand-new strand from scratch without needing a primer, and it tolerates a slightly higher error rate because RNA copies are temporary and disposable.
There is not just one kind. Bacteria get by with a single RNA polymerase for all their genes. Your eukaryotic cells run three main ones with a division of labor: RNA polymerase I makes most ribosomal RNA, RNA polymerase II makes the messenger RNA that codes for proteins (and many regulatory RNAs), and RNA polymerase III makes transfer RNA and other small RNAs. Pol II, the protein-coding workhorse, is the most heavily studied and most tightly controlled.
The antibiotic rifampicin, used against tuberculosis, works by jamming the bacterial RNA polymerase so the bacterium can no longer transcribe its genes. Because our human RNA polymerases are shaped differently, the drug leaves our cells alone.
Rifampicin kills bacteria by disabling their RNA polymerase while sparing ours.
RNA polymerase makes RNA, not DNA — do not confuse it with DNA polymerase, which copies DNA during replication. Also, RNA polymerase needs no primer to begin.