Initiator Recognition in a Primitive Eukaryote: IBP39, an Initiator-Binding Protein from Trichomonas vaginalis

Abstract

While considerable progress has been made in understanding the mechanisms of transcription in higher eukaryotes, transcription in single-celled, primitive eukaryotes remains poorly understood. Promoters of protein-encoding genes in the parasitic protistTrichomonas vaginalis, which represents one of the deepest-branching eukaryotic lineages, have a bipartite structure with gene-specific regulatory elements and a conserved core promoter encompassing the transcription start site. Core promoters in T. vaginalis appear to consist solely of a highly conserved initiator (Inr) element that is both a structural and a functional homologue of its metazoan counterpart. Using DNA affinity chromatography, we have isolated an Inr-binding protein from T. vaginalis. Cloning of the gene encoding the Inr binding protein identified a novel 39-kDa protein (IBP39). We show that IBP39 binds to both double and single Inr motifs found in T. vaginalisgenes and that binding requires the conserved nucleotides necessary for Inr function in vivo. Analyses of the cloned IBP39 gene revealed no homology at the protein sequence level with identified proteins in other organisms or the presence of known DNA-binding domains. The relationship between IBP39 and Inr-binding proteins in metazoa presents interesting evolutionary questions.

ACKNOWLEDGMENTS

We thank members of the Johnson and Smale laboratories for helpful advice and discussion.

This work was supported by a grant from the NIH (AI30537) to P.J.J., a USPHS predoctoral training award (GM07185) to D.R.L., and a USPHS postdoctoral training award (AI07323) to A.O.T.L. S.T.S. is an Investigator with the Howard Hughes Medical Institute. P.J.J. is the recipient of a Burroughs-Wellcome Scholar Award in Molecular Parasitology.