RNA methyltransferases Influence Noncoding RNA Biogenesis and Function Through Catalytic-Independent Activities

Continued advances in the high throughput detection of post-transcriptional RNA modifications have enabled large scale, mechanistic studies into the importance of RNA modifications in regulating the structure, function, and stability of coding and noncoding RNAs. While modifications themselves have a major role in influencing the fate of an RNA, recent evidence from bacteria suggests that RNA modification enzymes can possess non-catalytic functions that nevertheless contribute to RNA functionality. This dissertation aims to expand this idea to eukaryotic RNA modification enzymes, with a focus on uncovering the catalytic-independent functions of the fission yeast RNA methyltransferases Bmc1 and Trm1. Bmc1 is a homolog of the human methyl phosphate capping enzyme (MePCE), which has been well studied for its role in catalyzing the addition of a 5 -monomethyl phosphate cap on select RNA Polymerase III transcripts such as the 7SK snRNA. In answer to the long-standing question as to the function of an MePCE homolog in fission yeast, an organism with no known 7SK, this work revealed that Bmc1 assumes a non-catalytic role in the fission yeast telomerase enzyme by promoting holoenzyme assembly and telomerase RNA stability. Further analysis demonstrated that Bmc1 also interacts with the U6 snRNA to direct 2-O-methylation and influence formation of a U6-containing snRNP, and that neither of these activities requires Bmc1 catalytic activity. Finally, this work shows that the fission yeast tRNA methyltransferase Trm1 promotes tRNA functionality and structural stability even in the absence of catalysis, suggesting its function as a tRNA chaperone. Collectively, these studies provide evidence supporting the multi-faceted nature of eukaryotic RNA modification enzymes and underscores their importance in many fundamental biological processes including splicing, protein translation, and maintaining genome integrity.