A role for Ssu72 protein in poly(A)-independent termination of
transcription by RNA polymerase II.
Eric J. Steinmetz,
David A. Brow
Biomolecular Chemistry, University of Wisconsin, 1300
University Ave., Madison, WI 53706, USA
Termination by RNA polymerase
II is thought to be coupled to transcript cleavage and polyadenylation.
We recently described a poly(A)-independent pathway for termination by
RNA pol II, which is used for snoRNA and snRNA transcripts (Nature
413:327-31, 2001). The essential RNA-binding proteins Nrd1 and
Nab3 and the putative helicase Sen1 cooperate to form non-polyadenylated
3' ends in response to specific RNA elements. Nrd1 associates with the
carboxyl-terminal domain (CTD) of RNA pol II's largest subunit, and
efficient function of the Nrd1 pathway requires the CTD kinase Ctk1.
Interestingly, Nrd1 autoregulates synthesis of its mRNA through an
element in the 5'-UTR. We now report a role for the SSU72 gene
product in Nrd1-dependent 3'-end formation. SSU72 encodes an
essential TFIIB- and Pol II-interacting protein that was recently
reported to associate with the cleavage and polyadenylation machinery.
Two novel ssu72 ts mutations allow readthrough of a Nrd1-dependent snoRNA terminator and derepress Nrd1 mRNA synthesis,
indicating a positive role for Ssu72 protein in poly(A)-independent 3'-end formation. The two mutations, which map 9 amino acids apart within
the 206-residue Ssu72 protein, do not appear to affect cleavage and
polyadenylation. We are currently selecting additional mutations to
determine if they define a functional domain of Ssu72. We are also
testing the extent of overlap in the machinery for poly(A)-dependent and
-independent termination pathways.
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