RNA polymerases catalyse the DNA dependent polymerisation of RNA. Prokaryotes contain a single RNA polymerase compared to three in eukaryotes (not including mitochondrial. and chloroplast polymerases). This domain, domain 2, contains the active site. The invariant motif -NADFDGD- binds the active site magnesium ion [1,2].

1: Structural modules of the large subunits of RNA polymerase. Introducing archaebacterial and chloroplast split sites in the beta and beta' subunits of Escherichia coli RNA polymerase. Severinov K, Mustaev A, Kukarin A, Muzzin O, Bass I, Darst SA, Goldfarb A; J Biol Chem 1996;271:27969-27974. PMID:8910400

2: 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