Mapping regions of yeast transcription factor IIIA required for DNA binding, interaction with transcription factor IIIC, and transcription activity.

TFIIIA, a site-specific DNA-binding protein containing nine zinc finger motifs, is an RNA polymerase III transcription factor responsible for the assembly of a transcription complex on the 5 S RNA gene. We have analyzed the effect of deleting various regions of yeast TFIIIA on its ability to bind to the internal control region of the yeast 5 S RNA gene, to recruit TFIIIC into the TFIIIA.DNA complex and to promote in vitro transcription of the 5 S RNA gene. A truncated polypeptide containing only the three amino-terminal zinc fingers retained the ability to bind specifically to the internal control region of the 5 S RNA gene. The TFIIIA.DNA complex formed with this three zinc finger module was able to recruit TFIIIC but could not promote transcription. An 81-amino acid domain, which interrupts the repeating zinc finger motifs between fingers 8 and 9, was found to be essential for the transcription activity of the protein. Removal of the carboxyl-terminal region of yeast TFIIIA that extends beyond the ninth zinc finger motif had no effect on the in vitro activities of the protein. Extending the carboxyl-terminal deletion to include the ninth zinc finger led to reduced transcription activity. We speculate that the ninth zinc finger serves to anchor the carboxyl-terminal portion of TFIIIA on the 5 S RNA gene, correctly orienting the 81-amino acid domain to serve its role in promoting transcription. Removal of the region of the protein amino-terminal to the first zinc finger motif was without effect. Extending the amino-terminal deletion to include the first zinc finger appeared to decrease the affinity of TFIIIA for DNA without affecting the specificity of the protein-DNA interaction. The resultant TFIIIA.DNA complex was defective in recruiting TFIIIC and did not support transcription.

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