| Research Abstract |
During this research term, we have focused on the identification of new elements regulating onset of differentiation of fission yeast and isolated 4 such elements playing key roles regulating the onset of differentiation and switching growth and differentiation. One is phhl^+ encoding a stress MAP kinase highly homologous with mammalian p38. Analysis shows that phhl is required for nutrient starvation-invokes induction of Ste11, a key transcriptional factor essential for the onset of differentiation, providing a molecular basis for the promotion of differentiation by stress. The second is rcdl^+ whose structural homologues are present at least in budding yeast, plant, nematodes and humans. rcdl^+ is essential for nitrogen starvation-invoked differentiation and Ste11 induction. The human homologue of rcdl^+ is expressed abundantly in tests, ovary, spleen and thymus, where cell proliferation and differentiation are actively taking place. The third is nrdl^+ encoding a typical RNA binding
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protein. Analysis shows that the biological role of this gene is to inhibit differentiation by repressing Ste11-regulated genes essential for conjugation and/or meiosis until cells reach a critical point of starvation. We also isolated rat and human homologues of nrdl^+ by expression cloning using fission yeast as host. They are named ROD1. As far as assayd in fission yeast, ROD1 is functionally indistinguishable from nrdl^+. Overexpression of ROD1 in a human hematopietic cell line effectively blocks its differentiation to megakaryocytes.
The fourth is srwl^+ encoding a WD repeat protein. Cells lacking this gene are unable to start differentiation, poor to arrestin G1 and defectivein G2 control. The inability to start differentiation is suppressed by deletion of the cig2 cyclin gene, which we previously identified as a negative regulator of differentiation. Analysis shows that srwl^+ is essential for nutrient starvation-induced degradation of the Cdc13 mitotic cyclin. Thus, srwl^+ is a key factor switching between cell proliferation and differentiation. Less
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