Figure 6.

Mechanism of 5FU action. Summary of the major 5FU targets previously described in eukaryotic cells (pink boxes) and new potential ones predicted by our analysis in the fission yeast model (yellow boxes). 5FU is converted intracellularly into three main active metabolites: fluorodeoxyuridine monophosphate (FdUMP), fluorodeoxyuridine triphosphate (FdUTP) and fluorouridine triphosphate (FUTP). FdUMP affects DNA replication because it inhibits the thymidylate synthase causing a decrease in the production of dTMP. FdUTP misincorporated into DNA during replication interferes with the normal DNA metabolism. FUTP can be misincorporated into different types of RNA molecules directly involved in protein synthesis (mRNA, tRNA, rRNA, snRNA) or in other processes such as RNA-mediated heterochromatin formation (dg/dh transcripts) or transcription regulation (antisense transcripts). These 5FU-containing RNAs can undergo defective processing and post-transcriptional modifications affecting their normal functions. The new proposed 5FU targets in eukaryotic cells include heterochromatin, chromosome segregation and histone modification/exchange. When incorporated into centromeric transcripts, FUTP could alter the RNA-mediated heterochromatin formation. In S. pombe the RNA-directed RNA polymerase complex (RDRC) copies the centromeric transcripts into dsRNA that are further processed by Dicer (Dcr1) into siRNA. The RITS complex loads these siRNA molecules, which facilitates its positioning at heterochromatic regions. The Clr4 methyltransferase complex (CLRC) methylates H3K9 providing a platform to recruit proteins necessary for heterochromatin formation and spreading. Chromosome segregation is also affected in 5FU-treated cells maybe as an indirect consequence of heterochromatic defects.