ISOLATION AND CHARACTERIZATION OF A CDNA-ENCODING DROSOPHILA TRANSCRIPTION FACTOR-TFIIB

A Drosophila cDNA encoding a human transcription factor TFIIB homologu e was isolated by PCR methods. The deduced amino acid sequence indicat es 85% sequence similarity with human TFIIB, and the corresponding cDN A product expressed in Escherichia coli is interchangeable with human TFIIB for both basal and GAL4-VP16-induced transcription. Structural m otifs including the direct repeats, basic repeats, and sigma-sequence similarities are well conserved among Drosophila, human, and Xenopus T FIIB. However, the N-terminal region of each direct repeat is less con served among the three species, suggesting the presence of two structu ral subdomains in the direct repeat. Moreover, the amino acid changes in the N-terminal subdomain produce altered positions of the conserved amino acids between the direct repeats. An overall similarity in gene ral structural features between TFIIB and TFIID-tau (the TATA-binding subunit of TFIID) was previously noted. However, in contrast to the se quence divergence reported for the N-terminal domains of TFIID-tau fro m different species, the N-terminal sequence of TFIIB was highly conse rved among the species. This suggests that TFIIB has a more rigid stru cture, consistent with its function as a "bridging" protein between TF IID and RNA polymerase II. Further implications of the TFIIB structure are discussed.