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Figure 2.
Structure of the C-terminal domain of T. brucei TFIIB
(tTFIIB[C]) and comparison to human TFIIB[C]. (A) Structure of
tTFIIB[C] (this work). Each helix of the 5-helix cyclin fold is
colored identically in each of the 2 modules. Additional motifs
distinct in tTFIIB (H6, H2′A, H3′A, and H6′) are blue.
H2′A is behind H2 in this view. Dashed lines denote amino
acids not visible in the crystal, and the number of residues is
in parentheses. Arrows denote regions for comparison between
tTFIIB[C] and human TFIIB[C] in panel B and their position in
the sequence in panel C as discussed in the text: Helix H6 (blue
arrow) and the linker between H3′A and H4′ (black arrow).
(B) Structure of human TFIIB[C] (PDB ID 1c9b) (29). (C) The
amino acid sequence of tTFIIB[C] (Tb; accession no. EAN76636) is
aligned with that of T. cruzi (Tc; XP 806216), S. cerevisiae
(Sc; P29055), and the sequences of known TFIIB[C] structures
from P. woesei (Pw; 1d3u), and Homo sapiens (Hs; 1c9b). Sequence
gaps are denoted by dashes in panel C. tTFIIB[C] amino acid
numbers and secondary structure elements are indicated above the
alignment, and human TFIIB[C] secondary structure elements are
indicated below the alignment. Helices are depicted as boxes,
intervening segments as lines, gaps in the structural alignment
are blank, and residues not visible in the electron density are
denoted by dashes. Amino acids that were changed to alanine in
tTFIIB variants are indicated by asterisks (see Figs. 3 and 4).
Structurally equivalent residues in human TFIIB that affected
function when mutated are denoted by equal signs (19). Residues
Asp-229 to Thr-237 are omitted from the S. cerevisiae sequence
for clarity.
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