Human transcription factor TFIIIC2 specifically interacts with a unique sequence in the Xenopus laevis 5S rRNA gene

Transcription factor TFIIIC2 derived from human cells is required for tRNA-type gene transcription and binds with high affinity to the essential B-box promoter element of tRNA-type genes. Although 5S rRNA genes contain no homology with the tRNA-type gene B box, we show that TFIIIC2 is also required for Xenopus laevis 5S rRNA gene transcription. TFIIIC2 protected an approximately 30-base-pair (-10 to +18) region of a Xenopus 5S rRNA gene from DNase I digestion. This region, which spanned the transcription start site, included sequences that are highly conserved among eucaryotic 5S rRNA genes and have no homology with the B-box sequence of tRNA genes. Mutation of the TFIIIC2-binding site reduced transcription of the 5S rRNA gene by a factor of 10 in HeLa cell extracts. Methylation of C residues within the TFIIIC2-binding site interfered with binding of TFIIIC2. These results suggest a role of the TFIIIC2-binding sequence in 5S rRNA gene transcription. In addition, the 5S rRNA gene binding site and the tRNA-type gene B-box sequence did not compete with each other for binding to TFIIIC2 any better than did an unrelated DNA sequence, indicating that TFIIIC2 interacts with 5S rRNA genes and tRNA-type genes through separate DNA-binding domains or polypeptides.