The divergent eukaryote Trichomonas vaginalis has an m7G cap methyltransferase capable of a single N2 methylation.
Department
of Microbiology, Immunology, and Molecular Genetics, University of
California, Los Angeles, Los Angeles, CA 90095-1489, USA.
Abstract
Eukaryotic
RNAs typically contain 5' cap structures that have been primarily
studied in yeast and metazoa. The only known RNA cap structure in
unicellular protists is the unusual Cap4 on Trypanosoma brucei mRNAs. We
have found that T. vaginalis mRNAs are protected by a 5' cap structure,
however, contrary to that typical for eukaryotes, T. vaginalis
spliceosomal snRNAs lack a cap and may contain 5' monophophates. The
distinctive 2,2,7-trimethylguanosine (TMG) cap structure usually found
on snRNAs and snoRNAs is produced by hypermethylation of an m(7)G cap
catalyzed by the enzyme trimethylguanosine synthase (Tgs). Here, we
biochemically characterize the single T. vaginalis Tgs (TvTgs) encoded
in its genome and demonstrate that TvTgs exhibits substrate specificity
and amino acid requirements typical of an RNA cap-specific,
m(7)G-dependent N2 methyltransferase. However, recombinant TvTgs is
capable of catalysing only a single round of N2 methylation forming a
2,7-dimethylguanosine cap (DMG) as observed previously for Giardia
lamblia. In contrast, recombinant Entamoeba histolytica and Trypanosoma
brucei Tgs are capable of catalysing the formation of a TMG cap. These
data suggest the presence of RNAs with a distinctive 5' DMG cap in
Trichomonas and Giardia lineages that are absent in other protist
lineages.
