RNA polymerase

Each RNA
polymerase uses a different type of promoter.
RNA polymerase II cannot initiate transcription
without a complex of general transcription factors
(see B) that bind to a single upstream promoter
(1). RNA polymerase I (2) has a bipartite
promoter, one 170 to 180 bp upstream (5! direction)
and another from about 45 bp upstream to
20 bp downstream (3! direction). The latter is
called the core promoter. Pol I requires two ancillary
factors, UBF1 and SL1. SL1 consists of four
proteins including a TBP (see B) that cannot
bind directly to the promoter. It binds to UBF1,
after which Pol I can bind to the core promoter
to initiate transcription (2). RNA polymerase III
uses either upstream promoters or two internal
promoters downstream of the transcription
start site (3). Three transcription factors are required
with internal promoters, TFIIIA (a zinc
finger protein, see p. 218), TFIIIB (a TBP and two
other proteins), and TFIIIC (a large, more than
500 kDa protein).