| Ribosome biogenesis and cell cycle are coordinated processes (Du and Stillman, 2001 Bernstein and Baserga, 2004; Fatica and Tollervey, 2002; Li et al., 2009; Strezoska et al., 2002). Mutations in genes encoding factors that are involved in ribosome biogenesis cause defects in ribosomal RNA processing as well as cell cycle arrest. Recent studies with mammalian cell lines have shown that ribosome biogenesis is also linked to tumorgenesis, that is mutation or depletion of ribosomal factors, leads to cancer cell proliferation (Montanaro et al., 2008). The yeast Saccharomyces cerevisiae is a useful model organism for understanding the connections between ribosome biogenesis and cell cycle control. Only a handful of studies have been done and these have mainly focused on different transacting factors involved in ribosome biogenesis; few studies have focused on the roles of r-proteins themselves in linking cell cycle progression and rRNA processing. I wanted to investigate what roles these r-proteins may have in the progression of the cell cycle. My interest in this area was spurred by previous findings in our lab showing that depleting ribosomal protein RpL4 of S. cerevisiae results in defects in both rRNA processing and cell cycle- with accumulation of di- and tri-budded cells.;I analyzed strains constructed by Philip Milkereit's and John Woolford's labs in which the gene(s) for a given ribosomal protein were knocked out of the chromosome and a gene for that particular r-protein was placed under a galactose promoter (Pgal) either in the chromosome or a plasmid. In total, I analyzed constructs for twenty-nine 60S r-protein genes and twenty-seven 40S r-protein genes. To examine effects on the cell cycle of repression of the Pgal controlled gene, cells, were shifted to glucose medium, and the cultures were examined by flow cytometric analysis and sorting. Cells were also analyzed microscopically to look for abnormal cell morphology. I observed that depletion of r-proteins yields a variety of defects in DNA content and cell morphology, depending on which protein was repressed.;Depletion of RpL17 led to cells forming chains of varying lengths. I characterized this defect in more detail, analyzing, for example, spindle formation, chromosome segregation, and other cytological markers of the cell cycle. Ribosomal RNA processing and ribosome biogenesis for RpL17 were studied by Northern blots, sucrose gradients, pulse-chase analysis with radioactive uracil, and primer extension. These results showed that depletion of RpL17 leads to a defect in 27S processing and degradation of pre-existing ribosomes. My findings indicate that RpL17 plays a role in cell cycle progression, bud site-selection, ribosome biogenesis, and ribophagy, indicating a possibility that these pathways may be interconnected. |
