Loss of the RNA trimethylguanosine cap is compatible with nuclear accumulation of spliceosomal snRNAs but not pre-mRNA splicing or snRNA processing during animal development

The 2,2,7-trimethylguanosine (TMG) cap is synthesized by the conserved Tgs1 enzyme, which is abundantly present on snRNAs essential for pre-mRNA splicing. In our study, we characterize new mutations in the Drosophila tgs1 gene and studied their effects on development and on splicing. Utilizing a hypomorphic tgs1 mutation, we uncovered a prominent role of Tgs1 in pre-mRNA splicing, particularly of introns with smaller sizes and weaker splice sites. Remarkably, stronger tgs1 mutations generated by CRISPR-Cas9 cause lethality without disrupting splicing, likely due to the preponderance of a maternal contribution and stable TMG capped snRNAs maintaining a near normal level of TMG modified snRNAs, which further proved that the TMG modification is important in splicing. 8 samples in total; 4 kinds of RNA samples from wildtype and tgs1 mutant animals were prepared and sequenced with two replicates.