Abstract
TBP functions in transcription initiation in all eukaryotes and in Archaebacteria. Although the 181-amino acid (aa) carboxyl (C-) terminal core of the protein is highly conserved, TBP proteins from different phyla exhibit diverse sequences in their amino (N-) terminal region. In mice, the TBP N-terminus plays a role in protecting the placenta from maternal rejection; however the presence of similar TBP N-termini in nontherian tetrapods suggests that this domain also has more primitive functions. To gain insights into the pretherian functions of the N-terminus, we investigated its phylogenetic distribution. TBP cDNAs were isolated from representative nontetrapod jawed vertebrates (zebrafish and shark), from more primitive jawless vertebrates (lamprey and hagfish), and from a prevertebrate cephalochordate (amphioxus). Results showed that the tetrapod N-terminus likely arose coincident with the earliest vertebrates. The primary structures of vertebrate N-termini indicates that, historically, this domain has undergone events involving intragenic duplication and modification of short oligopeptide-encoding DNA sequences, which might have provided a mechanism of de novo evolution of this polypeptide.