Eukaryotic RNA Polymerases

There are two important differences between eukaryotes and prokaryotes:

• One is the existence of multiple RNA polymerases in eukaryotes. There are three RNA polymerases each responsible for transcribing a different set of genes as seen in the list below.

• Another is that in eukaryotes the RNA Polymerase does not bind directly to the promoter sequence. Instead, various Ttranscription Factors (TFs) bind to the promoter and these are then bound by RNAP. This is covered in detail under Initiation.

The genes that are transcribed by different RNA polymerases have different promoters and the process of initiation differs somewhat. However, the elongation reaction is the same for each RNAP.

• RNA Polymerase I: Transcribes all rRNA except 5S rRNA


• RNA Polymerase II: Transcribes nuclear pre-mRNA


• RNA Polymerase III: Transcribes tRNAs and 5S rRNA as well as various small RNAs