CHARACTERIZATION OF THE MAMMALIAN METHYLTRANSFERASE TGS1

Abstract

In eukaryotes, RNAs transcribed by RNA polymerase II are capped at their 5’ end. The cap is added co-transcriptionally and, in the case of mRNAs, contains one methyl group, resulting in a monomethylguanosine cap. In the case of some noncoding RNAs, two additional methyl groups are added, resulting in a 2,2,7-trimethylguanosine cap. TMG-capped RNAs include snRNAs (U1, U2, U4, U5, U7), snoRNAs (U3, U8, U13) and telomerase RNA. The enzyme responsible for the cap hypermethylation is methyltransferase Tgs1, initially identified as PIMT in human and mouse. Tgs1 deletion in yeasts results in mild cold-sensitive phenotype. However, early embryonic lethality has been reported in Tgs1-KO mice. We identified a mutant version of Tgs1 in the putative Tgs1-KO mouse cell line. Although the mutant Tgs1 retains its methyltransferase motif, it results in processing defect of U2 snRNA. To further understand the function of mammalian Tgs1, we have used genome editing to generate a human cell line deleted for endogenous Tgs1 and carrying a tetracycline-inducible version. Repression of human Tgs1 expression to undetectable level results in a severe growth phenotype in cell culture. The effects of Tgs1-depletion on RNA cap hypermethylation and gene expression were analyzed using the human conditional knockdown cell line. Our study provides new insights on a previously reported Tgs1-KO mouse model, generates new tools for characterization of mammalian Tgs1, and provides comprehensive information on the impact of Tgs1-depletion in mammalian cells.