title: A metazoan/plant-like capping enzyme and cap modified nucleotides in the unicellular eukaryote Trichomonas vaginalis.
abstract: The cap structure of eukaryotic messenger RNAs is initially elaborated through three enzymatic reactions: hydrolysis of the 5'-triphosphate, transfer of guanosine through a 5'-5' triphosphate linkage and N7-methylation of the guanine cap. Three distinctive enzymes catalyze each reaction in various microbial eukaryotes, whereas the first two enzymes are fused into a single polypeptide in metazoans and plants. In addition to the guanosine cap, adjacent nucleotides are 2'-O-ribose methylated in metazoa and plants, but not in yeast. Analyses of various cap structures have suggested a linear phylogenetic trend of complexity. These findings have led to a model in which plants and metazoa evolved a two-component capping apparatus and modification of adjacent nucleotides while many microbial eukaryotes maintained the three-component system and did not develop modification of adjacent nucleotides. Here, we have characterized a bifunctional capping enzyme in the divergent microbial eukaryote Trichomonas vaginalis using biochemical and phylogenetic analyses. This unicellular parasite was found to harbor a metazoan/plant-like capping apparatus that is represented by a two-domain polypeptide containing a C-terminus guanylyltransferase and a cysteinyl phosphatase triphosphatase, distinct from its counterpart in other microbial eukaryotes. In addition, T. vaginalis mRNAs contain a cap 1 structure represented by m(7)GpppAmpUp or m(7)GpppCmpUp; a feature typical of metazoan and plant mRNAs but absent in yeast mRNAs. Phylogenetic and biochemical analyses of the origin of the T. vaginalis capping enzyme suggests a complex evolutionary model where differential gene loss and/or acquisition occurred in the development of the RNA capping apparatus and cap modified nucleotides during eukaryote diversification.
authors.affiliation: Department of Microbiology, Immunology, and Molecular Genetics, University of California, Los Angeles, Los Angeles, California, United States of America.
authors.forename: Augusto
authors.initials: A
authors.lastname: Simoes-Barbosa
authors.forename: Robert P
authors.initials: RP
authors.lastname: Hirt
authors.forename: Patricia J
authors.initials: PJ
authors.lastname: Johnson
mesh_terms: D000818:Animals; D043583:Nucleoside-Triphosphatase; D009711:Nucleotides; D009713:Nucleotidyltransferases; D010802:Phylogeny; D012315:RNA Caps; D012333:RNA, Messenger; D014246:Trichomonas vaginalis / Q000235:genetics*
chemical_list: D009711:Nucleotides; D012315:RNA Caps; D012333:RNA, Messenger; D009713:Nucleotidyltransferases; C020842:guanylyltransferase; D043583:Nucleoside-Triphosphatase