SummarySpore formation is a common process in the developmental cycle of fungi. In the yeast Saccharomyces cerevisiae, Ime2 is a key protein kinase for the meiotic cell cycle, which precedes ascospore formation. Here, we analysed the IME2-related imeB gene of the filamentous ascomycete Aspergillus nidulans. imeB deletion strains are retarded in growth and overproduce fertile sexual fruiting bodies in the presence of light, which normally represses sexual development. imeB mutants also display abnormal differentiation of sexual Hülle cells in submerged cultures. Increased sexual development of imeB mutants is dependent on VeA, a component of the heterotrimeric velvet complex. A combined deletion of imeB with the phytochrome fphA, a red light receptor, results in a complete loss of light response, suggesting that ImeB and FphA cooperate in light-mediated inhibition of sexual development. Furthermore, we found that imeB mutants fail to produce the mycotoxin sterigmatocystin, an aflatoxin precursor, and show that ImeB is needed for expression of the sterigmatocystin gene cluster. ImeB contains a TXY motif conserved in mitogen-activated protein kinases. This sequence element is essential for ImeB function. We conclude that ImeB is a mitogen-activated protein kinaserelated protein kinase required for the co-ordinated control of light-dependent development with mycotoxin production.

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Proteolytic destruction of many cyclins is induced by a multisubunit ubiquitin ligase termed the anaphase promoting complex/cyclosome (APC/C). In the budding yeast Saccharomyces cerevisiae, the S phase cyclin Clb5 and the mitotic cyclins Clb1-4 are known as substrates of this complex. The relevance of APC/C in proteolysis of Clb5 is still under debate. Importantly, a deletion of the Clb5 destruction box has little influence on cell cycle progression. To understand Clb5 degradation in more detail, we applied in vivo pulse labeling to determine the half-life of Clb5 at different cell cycle stages and in the presence or absence of APC/C activity. Clb5 is significantly unstable, with a half-life of ϳ8 -10 min, at cell cycle periods when APC/C is inactive and in mutants impaired in APC/C function. A Clb5 version lacking its cyclin destruction box is similarly unstable. The half-life of Clb5 is further decreased in a destruction boxdependent manner to 3-5 min in mitotic or G 1 cells with active APC/C. Clb5 instability is highly dependent on the function of the proteasome. We conclude that Clb5 proteolysis involves two different modes for targeting of Clb5 to the proteasome, an APC/C-dependent and an APC/C-independent mechanism. These different modes apparently have overlapping functions in restricting Clb5 levels in a normal cell cycle, but APC/C function is essential in the presence of abnormally high Clb5 levels.

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To my parents Hüsnü & Saziye Sari iv Acknowledgement I am thankful to Gerhard Braus who gave me the opportunity to do my thesis in his department.Further, I want to thank Stefan Irniger, specially, for his excellent supervision during my PhD work. He had always good ideas which pushed my work always forward.Partick Dieckhoff, Melanie Bolte, Wiebke Meyer, and Dipak Maskey, and the two internship students Tülin Ersahin and Sevim Dalva contributed a lot for the success of this study. Especially Patrick Dieckhoff, who prepared me for the PhD work, I want to thank for his calm and friendly mentoring during my diploma thesis.To all members of the department-the Aspergillus group, the Verticillium group, the Neurospora group, and the yeast group-I want to thank for the warm working atmosphere, all of them always being friendly and helpful. Also for the help of the the two technicians in Hannover, Sybille Traupe and Armgard Janczikowski, I am thankfull.Of course I have to thank Özgür Bayram, who was always a precious friend and helped me a lot with the Aspergillus project. His support and contributions especially during the last year of my PhD work guaranteed the success of the Aspergillus project.The chats with Ingrid Bahr, Özgür Bayram, Marcia Kress, and the other lab colleques had been very relaxing during the day of work.For the relaxing atmosphere after work I want to thank my wife Ruhsan Sari. It was always restorative to play with my two sons, Muhammed Enes and Yusuf Emin, on weekends, although they always forced me to look spongebob on tv. All my accumulated frustation during the week, I could kick off on weekends. Special thanks to my soccer club AY YILDIZ! I want to thank my father, Hüsnü Sari, for the discipline he taught me and my mother, Saziye Sari, for the love she gave me. Finally, I want to thank my sisters Sultan and Elmas and my brother Mehmet, that I was never alone.