RNA polymerase promoters

Eukaryotic cells contain three RNA polymerases
(Pol I, Pol II, and Pol III). Pol I is located in the
nucleolus, synthesizes ribosomal RNA, and accounts
for about 50–70% of the relative activity.
Pol II and Pol III are located in the nucleoplasm
(the part of the nucleus excluding the nucleolus).
Pol II represents 20–40% of cellular activity.
It is responsible for the synthesis of heterogeneous
nuclear RNA (hnRNA), the precursor of
mRNA. Pol III, responsible for the synthesis of
tRNAs and other small RNAs, contributes only a
minor activity of about 10%. Each of the large
eukaryotic RNA polymerases (500 kDa or more)
is more complex, with 8–14 subunits, than the
single prokaryotic RNA polymerase.