Trimethylguanosine (TMG) cap structures are characteristic of small nuclear (sn) RNAs, small nucleolar (sno) RNAs, and telomerase RNA. TMG is formed post-transcriptionally by the enzyme Tgs1, which catalyzes two successive methyl additions to the N2 atom of the m7G cap (Mouaikel et al. 2002; Hausmann and Shuman 2005; Chang et al. 2010). Whereas m7G caps are essential for the viability of eukaryal cells, TMG caps are not (Mouaikel et al. 2002; Hausmann et al. 2007). Saccharomyces cerevisiae and Schizosaccharomyces pombe tgs1∆ cells have no detectable TMG caps on their snRNAs, snoRNAs, or telomerase RNA, signifying that there is no Tgs1-independent route to form TMG caps (Mouaikel et al. 2002; Hausmann et al. 2007; Gallardo et al. 2008; Franke et al. 2008; Simoes-Barbosa et al. 2012). tgs1∆ yeast cells display apparently normal steady-state snRNA levels (Mouaikel et al. 2002; Hausmann et al. 2007) and no overt aberrations in the RNA or protein contents of their spliceosomal snRNPs, except for the acquisition of the nuclear cap-binding complex (CBC) as a stoichiometric component of the U1 snRNP (Schwer et al. 2011).