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In eukaryotes the initiation of transcription of protein encoding genes by
polymerase II (Pol II) is modulated by general and specific transcription
factors. The general transcription factors operate through common promoters
elements (such as the TATA box). At least eight different proteins associate
to form the general transcription factors: TFIIA, -IIB, -IID, -IIE, -IIF,
-IIG, -IIH and -IIS .
During mRNA elongation, Pol II can encounter DNA sequences that cause reverse
movement of the enzyme. Such 'backtracking' involved extrusion of the RNA
3'-end into the pore, and can lead to transcriptional arrest. Escape from
arrest requires cleavage of the extruded RNA with the help of TFIIS . TFIIS
is a protein of about 300 amino acids. It contains two conserved regions, a
central domain which is required for Pol II binding and a C-terminal C4-type
zinc finger essential for RNA cleavage. The zinc finger folds in a
conformation termed a 'zinc ribbon' (see <PDB:1TFI>)  characterized by a
three-stranded antiparallel β-sheet and two β-hairpins. A backbone model
for Pol II-TFIIS complex was obtained from X-ray analysis. It shows that a
β hairpin protrudes from the zinc finger and complements the pol II active
The TFIIS zinc finger is also found in the following proteins:
Eukaryotic and archebacterial RNA polymerase subunits of the 15 Kd / M
family (see <PDOC00790>).
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