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Pattern formation in DrosophilaThe establishment of polarity along the two major axes of the Drosophila embryo is one of the first steps in pattern formation. Genetic experiments indicate that the final identity of each cell along the axes is dependent on interaction of maternal and zygotic gene products. The anterior-posterior and dorsal-ventral polarity is initiated in the egg chamber during oogenesis and transmitted to the early embryo by maternal gene products. A signal transduction pathway leads to the asymmetric activation of zygotic genes along the dorsal-ventral axis. The anterior-posterior axis is established by the localization of determinants at the anterior and posterior ends of the embryo and these determinants regulate the expression of zygotic genes along the axis. Our work is focused on two genes that function in these processes of early pattern formation. The dorsal gene encodes the last step in the maternal signal transduction pathway that results in dorsal-ventral polarity. Dorsal is the ventral morphogen that instructs ventral and lateral cells as to their identity. and is a member of the Rel family of transcription factors that also includes the lymphocyte transcription factor NF-kB. the viral oncogene v-rel. and v-rel's cellular homolog. c-rel. The activity of dorsal is regulated posttranslationally. It is found in an inactive form in the cytoplasm of early embryos and is present in a ventral-to-dorsal nuclear gradient in blastoderm stage embryos. Once in the nucleus. dorsal protein acts as a transcriptional activator as well as a repressor of different zygotic genes within the same nucleus. Our work on dorsal concentrates on two major questions. How is the dorsal nuclear protein gradient established. and how does dorsal protein function once it is in the nucleus? We are studying the importance of phosphorylation of dorsal protein for its function. and we are using biochemical and genetic approaches to isolate new genes that function in the establishment of the dorsal nuclear gradient. Genetic studies show that the maternal Bicaudal-D gene product is involved in localizing determinants at the posterior end of the early embryo and that it is also essential for the differentiation of the oocyte early in oogenesis. most likely by localizing oocyte determinants as RNAs in the prospective oocyte. We have shown that both the Bicaudal-D RNA and protein accumulate in the prospective oocyte and that the localization of the Bicaudal-D and other RNAs in the oocyte is dependent on the accumulation of Bicaudal-D protein. The Bicaudal-D protein shows similarity to myosin heavy chain tails. paramyosin. and kinesin. and so may be involved in transporting RNAs to specific domains of the oocyte. We are investigating these possibilities by identifying proteins associated with Bicaudal-D genetically and biochemically. We are also determining the function of the localization of the RNA and protein and studying which sequences are necessary for the localization. A third project focuses on nuclear migration. We have cloned the Drosophila homolog of the human Lis-1 gene. that is affected in children with Lisencephaly. a strong disorder in brain development. In Aspergillus the Lis-1 homolog. NudF. has been isolated as a mutation in which nuclear migration is abolished. We plan to use Drosophila genetics to investigate different aspects of nuclear migration and its possible involvement in cell migration. Selected PublicationsKarachentsev D, Druzhinina M, Steward R. (2007) Free and chromatin-associated mono-, di-, and trimethylation of histone H4-lysine 20 during development and cell cycle progression. Dev Biol. 304(1):46-52. Sakaguchi A, Steward R. (2007) Aberrant monomethylation of histone H4 lysine 20 activates the DNA damage checkpoint in Drosophila melanogaster. J Cell Biol. 176(2):155-62. Minakhina S. Steward R. (2006) Melanotic mutants in Drosophila: pathways and phenotypes. Genetics. Jul 2; [Epub ahead of print] Minakhina S. Myers R. Druzhinina M. Steward R. (2005) Crosstalk between the actin cytoskeleton and Ran-mediated nuclear transport. BMC Cell Biol. 6:32. Siller KH. Serr M. Steward R. Hays TS. Doe CQ. (2005) Live imaging of Drosophila brain neuroblasts reveals a role for Lis1/dynactin in spindle assembly and mitotic checkpoint control. Mol Biol Cell. 16(11):5127-40. Minakhina S. Steward R. (2005) Axes formation and RNA localization. Curr Opin Genet Dev. 15(4):416-21. Yu B. Yang Z. Li J. Minakhina S. Yang M. Padgett RW. Steward R. Chen X. (2005) Methylation as a crucial step in plant microRNA biogenesis. Science. 307(5711):932-5. Karachentsev D. Sarma K. Reinberg D. Steward R. (2005) PR-Set7-dependent methylation of histone H4 Lys 20 functions in repression of gene expression and is essential for mitosis. Genes Dev. 19(4):431-5. Minakhina S. Yang J. Steward R. (2003) Tamo selectively modulates nuclear import in Drosophila. Genes Cells. Apr;8(4):299-310. Rice JC. Nishioka K. Sarma K. Steward R. Reinberg D. Allis CD. (2002) Mitotic-specific methylation of histone H4 Lys 20 follows increased PR-Set7 expression and its localization to mitotic chromosomes. Genes Dev. 16(17):2225-30. Nishioka K. Rice JC. Sarma K. Erdjument-Bromage H. Werner J. Wang Y. Chuikov S. Valenzuela P. Tempst P. Steward R. Lis JT. Allis CD. Reinberg D. (2002) PR-Set7 is a nucleosome-specific methyltransferase that modifies lysine 20 of histone H4 and is associated with silent chromatin. Mol Cell. 9(6):1201-13. Bhattacharya A. Steward R. (2002) The Drosophila homolog of NTF-2. the nuclear transport factor-2. is essential for immune response. EMBO Rep. 3(4):378-83. Oh. J.Y.. and Steward. R. (2001) Bicaudal-D is essential for egg chamber formation and cytosketal organization in Drosophila oogenesis. Developmental Biology 232:91-104 Drier. E.A.. Govind. S.. and Steward. R. (2000) Cactus-independent regulation of Dorsal nuclear import by the ventral signal. Current Biology 10:23-26. Oh. J.Y.. Baksa K.. and Steward. R. (2000) Functional domains of the Bicaudal-D protein. Genetics 154:713-724. Fernandez. R.. Takahashi. F.. Liu. Z.. Steward. R.. Stein. D.. and Stanley. E.R. (2000) The Drosophila Shark tyrosine kinase is required in both leading edge and lateral epidermis for embryonic dorsal closure. Genes & Development 14:604-14. Lin. P.-H.. Huang. L. H.. and Steward. R. (2000) Cactin. a conserved protein that interacts with the Drosophila IkB protein Cactus and modulates its function. Mech Dev. 94:57-65. Munn. K.. and Steward. R. (2000) The shut-down gene of Drosophila melanogaster encodes a novel FK506-binding protein essential for the formation of the germline cysts during oogenesis. Genetics 156:245-56. Liu. Z.. Steward. R.. and Luo. L. (2000) Drosophila Lis1 is required for neuroblast proliferation . dendritic elaboration and axonal transport. Nature Cell Biol. 2:776-788. |
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