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Schlagwörter:
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Zusammenfassung:
During gene transcription, the RNA polymerase (Pol) active
center can catalyze RNA cleavage. This intrinsic cleavage activity
is strong for Pol I and Pol III but very weak for Pol II. The
reason for this difference is unclear because the active centers of
the polymerases are virtually identical. Here we show that Pol II
gains strong cleavage activity when the C-terminal zinc ribbon
domain (C-ribbon) of subunit Rpb9 is replaced by its counterpart
from the Pol III subunit C11. X-ray analysis shows that the
C-ribbon has detached from its site on the Pol II surface and is
mobile. Mutagenesis indicates that the C-ribbon transiently
inserts into the Pol II pore to complement the active center. This
mechanism is also used by transcription factor IIS, a factor that
can bind Pol II and induce strong RNA cleavage. Together with
published data, our results indicate that Pol I and Pol III contain
catalytic C-ribbons that complement the active center, whereas
Pol II contains a non-catalytic C-ribbon that is immobilized on
the enzyme surface. Evolution of the Pol II system may have
rendered mRNA transcript cleavage controllable by the dissociable
factor transcription factor IIS to enable promoter-proximal
gene regulation and elaborate 3-processing and transcription
termination.
