Tgs1 is the enzyme responsible for converting 7-methyl-guanosine RNA caps to the 2,2,7-trimethylguanosine cap structures of small nuclear and small nucleolar RNAs. In comparison to budding yeast S.cerevisiae and fission yeast S.pombe encode a single Tgs1 protein, the primitive eukaryote Giardia lamblia encodes two paralogs, Tgs1 and Tgs2. Tgs2 is a monomeric enzyme that catalyzes methyl transfer from AdoMet to m7GDP to form m2,7GDP. Tgs2 also methylates m7GTP and m7GpppA but is unreactive with GDP, GTP, GpppA, ATP, CTP, or UTP. Tgs2's sp-68, Glu-91, and Trp-143 are
essential for Tgs2 methyltransferase activity in vitro (Hausmann et al. 2007 ).