Analysis of a ubiquitous promoter element in a primitive eukaryote: Early evolution of the initiator element

Typical metazoan core promoter elements, such as TATA boxes and Inr motifs, have pet to be identified in early-evolving eukaryotes, underscoring the e xtensive divergence of these organisms. Towards the identification of core promoters in protists, we have studied transcription of protein-encoding ge nes in one of the earliest-diverging lineages of Eukaryota, that represente d by the parasitic protist Trichomonas vaginalis. A highly conserved elemen t, comprised of a motif similar to a metazoan initiator (Inr) element, surr ounds the start site of transcription in all examined T. vaginalis genes. I n contrast, a metazoan-like TATA element appears to be absent in trichomona d promoters. We demonstrate that the conserved motif found in T. vaginalis protein-encoding genes is an Inr promoter element. This trichomonad Inr is essential for transcription, responsible for accurate start site selection, and interchangeable between genes, demonstrating its role as a core promot er element. The sequence requirements of the trichomonad Inr are similar to metazoan Inrs and can be replaced by a mammalian Inr. These studies show t hat the Inr is a ubiquitous, core promoter element for protein-encoding gen es in an early-evolving eukaryote. Functional and structural similarities b etween this protist Inr and the metazoan Inr strongly indicate that the Inr promoter element evolved early in eukaryotic evolution.