TRANSCRIPTION FACTOR-IID IN THE ARCHAEA - SEQUENCES IN THE THERMOCOCCUS CELER GENOME WOULD ENCODE A PRODUCT CLOSELY-RELATED TO THE TATA-BINDING PROTEIN OF EUKARYOTES

The first step in transcription initiation in eukaryotes is mediated b y the TATA-binding protein, a subunit of the transcription factor IID complex. We have cloned and sequenced the gene for a presumptive homol og of this eukaryotic protein from Thermococcus celer, a member of the Archaea (formerly archaebacteria). The protein encoded by the archaea l gene is a tandem repeat of a conserved domain, corresponding to the repeated domain in its eukaryotic counterparts. Molecular phylogenetic analyses of the two halves of the repeat are consistent with the dupl ication occurring before the divergence of the archaeal and eukaryotic domains. In conjunction with previous observations of similarity in R NA polymerase subunit composition and sequences and the finding of a t ranscription factor IIB-like sequence in Pyrococcus woesei (a relative of T. celer) it appears that major features of the eukaryotic transcr iption apparatus were well-established before the origin of eukaryotic cellular organization. The divergence between the two halves of the a rchaeal protein is less than that between the halves of the individual eukaryotic sequences, indicating that the average rate of sequence ch ange in the archaeal protein has been less than in its eukaryotic coun terparts. To the extent that this lower rate applies to the genome as a whole, a clearer picture of the early genes (and gene families) that gave rise to present-day genomes is more apt to emerge from the study of sequences from the Archaea than from the corresponding sequences f rom eukaryotes.