RNA polymerases catalyse the DNA-dependent polymerisation of RNA. Prokaryotes contain a single RNA polymerase compared with three in eukaryotes (not including mitochondrial or chloroplast polymerases). This domain in prokaryotes spans the gap between domains 4 and 5 of the yeast protein. It is also known as the external 1 region of the polymerase and is bound in association with the external 2 region [1].

1: Structural basis of transcription: RNA polymerase II at 2.8 angstrom resolution. Cramer P, Bushnell DA, Kornberg RD; Science 2001;292:1863-1876. PMID:11313498