A metazoan/plant-like capping enzyme and cap modified nucleotides in the unicellular eukaryote trichomonas vaginalis

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 m7gpppampup or m7gpppcmpup; 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. © 2010 simoes-barbosa et al.