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Mol. Cell. Biol., Mar 1995, 1455-1466, Vol 15, No. 3
VL Gerlach, SK Whitehall, EP Geiduschek and DA Brow
The Saccharomyces cerevisiae U6 RNA gene (SNR6), which is transcribed by
RNA polymerase III, has an unusual combination of promoter elements: an
upstream TATA box, an intragenic A block, and a downstream B block. In tRNA
genes, the A and B blocks are binding sites for the transcription
initiation factor TFIIIC, which positions TFIIIB a fixed distance upstream
of the A block. However, in vitro transcription of SNR6 with purified
components requires neither TFIIIC nor the A and B blocks, presumably
because TFIIIB recognizes the upstream sequences directly. Here we
demonstrate that TFIIIB placement on SNR6 in vivo is directed primarily by
the TFIIIC-binding elements rather than by upstream sequences. We show that
the A block is a stronger start site determinant than the upstream
sequences when the two are uncoupled by an insertion mutation. Furthermore,
while TFIIIC-independent in vitro transcription of SNR6 is highly sensitive
to TATA box point mutations, in vivo initiation on SNR6 is only marginally
sensitive to such mutations unless the A block is mutated. Intriguingly, a
deletion downstream of the U6 RNA coding region that reduces A-to-B block
spacing also increases in vivo dependence on the TATA box. Moreover, this
deletion results in the appearance of micrococcal nuclease- hypersensitive
sites in the TFIIIB chromatin footprint, indicating that TFIIIB binding is
disrupted by a mutation 150 bp distant. This and additional chromatin
footprinting data suggest that SNR6 is assembled into a nucleoprotein
complex that facilitates the TFIIIC-dependent binding of TFIIIB.
Copyright © 1995, American Society for Microbiology
TFIIIB placement on a yeast U6 RNA gene in vivo is directed primarily by TFIIIC rather than by sequence-specific DNA contacts
Department of Biomolecular Chemistry, University of Wisconsin Medical School, Madison 53706-1532.
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