<p> The task of transcribing nuclear genes is shared between three RNA polymerases in eukaryotes: RNA polymerase (pol) I synthesizes the large rRNA, pol II synthesizes mRNA and pol III synthesizes tRNA and 5S rRNA [<cite idref="PUB00028105"/>]. Pol I transcription is localised to discrete sites called nucleoli; these can be likened to ribosomefactories, in which rRNA is synthesised by pol I in the fibrillar centres and then processed and assembled into ribosomes inthe surrounding granular regions [<cite idref="PUB00028106"/>]. Prokaryotes, in contrast, posses a single RNA polymerase, with transcription being controlled by the particular signam factor interacting with the catalytic core.</p><p>This entry describes an N-terminal conserved region which can be found in the largest subunits of prokaryoptic and eukaryotic RNA polymerases.</p>