MOLECULAR-CLONING OF THE TRANSCRIPTION FACTOR TFIIB HOMOLOG FROM SULFOLOBUS-SHIBATAE

The Archaea (archaebacteria) constitute a group of prokaryotes that ar e phylogenetically distinct from Eucarya (eukaryotes) and Bacteria (eu bacteria). Although Archaea possess only one RNA polymerase, evidence suggests that their transcriptional apparatus is similar to that of Eu carya. For example, Archaea contain a homolog of the TATA-binding prot ein which interacts with the TATA-box like A-box sequence upstream of many archaeal genes. Here, we report the cloning of a Sulfolobus shiba tae gene that encodes a protein (transcription factor TFB) with striki ng homology to the eukaryotic basal transcription factor TFIIB, We sho w by primer extension analysis that transcription of the S. shibatae T FB gene initiates 27 bp downstream from a consensus A-box element, Sig nificantly, S. shibatae TFB contains an N-terminal putative metal-bind ing region and two imperfect direct repeats-structural features that a re well conserved in eukaryotic TFIIBs. This suggests that TFB may per form analogous functions in Archaea and Eucarya. Consistent with this, we demonstrate that S. shibatae TFB promotes the binding of S. shibat ae TBP to the A-box element of the Sulfolobus 16S/23S rRNA gene. Final ly, we show that S. shibatae TFB is significantly more related to TFB of the archaeon Pyrococcus woesei than it is to eukaryotic TFIIBs. The se data suggest that TFB arose in the common archaeal/eukaryotic ances tor and that the lineages leading to P. woesei and S. shibatae separat ed after the divergence of the archaeal and eukaryotic lines of descen t.