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Abstract:
RNA polymerase (Pol) I contains a 10-subunit catalytic
core that is related to the core of Pol II and
includes subunit A12.2. In addition, Pol I contains
the heterodimeric subcomplexes A14/43 and A49/
34.5, which are related to the Pol II subcomplex
Rpb4/7 and the Pol II initiation factor TFIIF, respectively.
Here we used lysine-lysine crosslinking, mass
spectrometry (MS) and modeling based on five crystal
structures, to extend the previous homology
model of the Pol I core, to confirm the location of
A14/43 and to position A12.2 and A49/34.5 on the
core. In the resulting model of Pol I, the C-terminal
ribbon (C-ribbon) domain of A12.2 reaches the
active site via the polymerase pore, like the
C-ribbon of the Pol II cleavage factor TFIIS, explaining
why the intrinsic RNA cleavage activity of Pol I is
strong, in contrast to the weak cleavage activity of
Pol II. The A49/34.5 dimerization module resides on
the polymerase lobe, like TFIIF, whereas the A49
tWH domain resides above the cleft, resembling
parts of TFIIE. This indicates that Pol I and also
Pol III are distantly related to a Pol II–TFIIS–TFIIF–
TFIIE complex.
