Katrine Vyff Løvschal:
Regulation of Ribonucleotide Reductase and Screening for Species-Specific Inhibitors in Fission Yeast

Date: 21-03-2019    Supervisor: Christian Holmberg

Cellular proliferation and genome integrity depend on accurate DNA replication and repair of damaged DNA, which in turn are impacted by the level of deoxyribonucleotides. Ribonucleotide reductase is the enzyme responsible for the de novo synthesis of deoxyribonucleotides in all organisms and is thus indispensible for life. The key requirement for ribonucleotide reductase has made it an attractive target for both anti-cancer and anti-microbial therapy, and during the last few decades, a considerable amount of effort has been devoted to developing novel and more specific inhibitors of this enzyme. Thorough knowledge of ribonucleotide reductase, and the mechanisms that interlock to restrain it, may lead to new rationales for developing novel inhibitors. Here, we have used fission yeast as model organism to explore how human ribonucleotide reductase can complement for lack of the yeast enzyme, and to explore the genetic interactions involving ribonucleotide reductase and genome stability factors, which are highly conserved from yeast to humans. We have demonstrated good cross-species complementation of the human enzyme regarding the production of the DNA precursors and cellular growth. Other functions were shown to be less complemented leading to checkpoint dependency and meiotic defects, independently of deoxyribonucleotide pools. Our results indicate that fission yeast ribonucleotide reductase has another cellular function, in addition to generating deoxyribonucleotides, and that the small protein inhibitor Spd1 serve as a negative regulator of both functions. Furthermore, our results provide evidence that Spd1 is able to restrain the human R1-R2 holoenzyme to regulate deoxyribonucleotide pools.

We also present a strategy to identify novel and species-specific inhibitors of ribonucleotide reductase using fission yeast as selective system. Based on preliminary experiments, we evaluate this strategy to have promising potential to identify novel inhibitors that may be used to treat various human diseases.