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 yet to be identified in early-evolving eukaryotes, underscoring the extensive divergence of these organisms. Towards the identification of core promoters in protists, we have studied transcription of protein-encoding genes in one of the earliest-diverging lineages of Eukaryota, that represented by the parasitic protist Trichomonas vaginalis. A highly conserved element, comprised of a motif similar to a metazoan initiator (Inr) element, surrounds the start site of transcription in all examined T. vaginalis genes. In contrast, a metazoan-like TATA element appears to be absent in trichomonad 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 promoter element. The sequence requirements of the trichomonad Inr are similar to metazoan Inrs and can be replaced by a mammalian Inr. These studies show that the Inr is a ubiquitous, core promoter element for protein-encoding genes in an early-evolving eukaryote. Functional and structural similarities between this protist Inr and the metazoan Inr strongly indicate that the Inr promoter element evolved early in eukaryotic evolution.