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Abstract
Hmt1 is the predominant arginine methyltransferase in yeast, involved in biological processes such as transcription, transcriptional regulation, nucleocytoplasmic transport of proteins and mRNAs and RNA splicing. Hmt1 is also known to mediate protein-protein and protein-RNA interactions. Despite its well-studied role in regulatory processes, the effects of Hmt1 knockout on the transcriptome has never been performed. This thesis investigated the changes in the transcriptome upon deletion of Hmt1, and through this, aimed to uncover as yet undiscovered biological functions of Hmt1. RNA-seq was used to compare the transcriptomic profiles of wild type versus Hmt1-null cells. In mid-log exponential growth, we found dysregulation of phosphate metabolism and polyphosphate accumulation in the Hmt1-null strain. Acid phosphatases (PH05, PH011 and PH012), phosphate transporters (PH084 and PH089) and the vacuolar transporter chaperone VTCJ were amongst genes significantly downregulated in hmt1/J.. Concomitant decreases were observed in intracellular polyphosphate levels and extracellular phosphatase activity. We demonstrated that transcription factor Pho4, responsible for activation of the phosphate regulatory pathway was methylated by Hmt1 in vitro at Arg-241. Genomic mutation of Arg- 241 to Lys in wild type produced to the same phenotype as hmt1/J.. This is the first study to establish a link between arginine methylation and phosphate regulation. As polyphosphate is a source of phosphate, ATP and energy in the yeast cell, we extended our investigation to cells in quiescence. RNA-seq revealed more than 50% genes were differentially expressed between hmt1/J. and wild type quiescent cells. Crucially, phosphate transport was still defective as the membrane-embedded phosphate transporters PH084, PH087, PH089 and PH090, vacuolar polyphosphate transporter PH091 and exopolyphosphatase PPX1 showed significant downregulation in hmt1/J. in quiescence. As a result of this dysregulation in phosphate importation and polyphosphate accumulation in hmt1/J., the Hmt1 deletion strain exhibited advanced quiescence as compared to wild type. A number of models are proposed to explain these observations.