The Dorsal morphogen acts as both an activator and a repressor of transcription in the Drosophila embryo to regulate the expression of dorsal/ventral patterning genes. Circumstantial evidence has suggested that Dorsal is an intrinsic activator and that additional factors (corepressors) convert it into a repressor. These corepressors, however, have previously eluded definitive identification. We show here, via the analysis of embryos lacking the maternally encoded Groucho corepressor and via protein-binding assays, that recruitment of Groucho to the template by protein:protein interactions is required for the conversion of Dorsal from an activator to a repressor. Groucho is therefore a critical component of the dorsal/ventral patterning system.

Modulation of transcription factor activity leading to changes in cell behavior (e.g., differentiation versus proliferation) is one of the critical outcomes of receptor tyrosine kinase (RTK) stimulation. In the early Drosophila embryo, activation of the torso (tor) RTK at the poles of the embryo activates a phosphorylation cascade that leads to the spatially specific transcription of the tailless (tll) gene. Our analysis of the tor response element (tot-RE} in the tll promoter indicates that the key activity modulated by the tor RTK pathway is a repressor present throughout the embryo. We have mapped the tor-RE to an ll-bp sequence; using this sequence as the basis for protein purification, we have determined that the proteins GAGA and NTF-1 (also known as Elf-l, product of the grainyhead gene) bind to the tot-RE. We demonstrate that NTF-1 can be phosphorylated by MAPK (mitogen-activated protein kinase), and that tll expression is expanded in embryos lacking maternal NTF-1 activity; these results make NTF-1 a likely target for modulation by the tor RTK pathway in vivo. The data presented here support a model in which activation of the tor RTK at the poles of the embryos leads to inactivation of the repressor and therefore, to transcriptional activation (by activators present throughout the embryo) of the tll gene at the poles of the embryo.[Key Words: Torso tyrosine kinase receptor; transcription factor; GAGA; NTF-1; Elf-l; tailless; Trithorax-like; grainyhead] Received June 20, 1995; revised version accepted October 24, 1995.A multitude of extracellular signals can lead to a change in the program of cellular gene activity. An important mechanism by which extracellular signals {growth factors) are transduced is by receptor tyrosine kinases (RTKs). Ligand stimulation of an RTK leads to activation of a series of proteins: Transient generation of Ras-GTP leads to activation of a cascade of serine-threonine kinases in the order Raf-1, mitogen-associated protein (MAP) kinase kinase (MAPKK), and MAP kinase {MAPK) (for review, see Hill and Treisman 1995). This "cassette" of sequentially acting proteins functions in mammalian cultured cells in the platelet-derived growth factor (PDGF), epidermal growth factor (EGF), and nerve growth factor (NGFI activated pathways, in Drosophila in the sevenless and torso receptor-activated pathways, and in nematodes in the Let-23 (an EGF receptor homolog)-activated pathway (for review, see Perrimon 1993).Modification of the transcriptional program resulting from RTK activation can be ascribed, at least in part, to the function of activated MAPK in phosphorylating vat4Corresponding author.ious ets family transcription factors (for review, see Hill and Treisman 1995). Although our understanding of the cassette of proteins involved in the phosphorylation cascade is fairly robust, the subsequent steps that ultimately lead to altered cell behavior are less well defined. Neither all of the transcription factors modulated, nor all of the genes targeted by any one activated RTK are known.T...

(Steward et al. 1988;Roth et al. 1989;Rushlow et al. 1989;Steward 1989). At high concentrations, Dorsal activates twist and snail in the most ventral region of the embryo, which gives rise to the mesoderm (Jiang et al. 1991;Pan et al. 1991;Thisse et al. 1991;Ip et al. 1992a). Intermediate concentrations of Dorsal allow the expression of rhomboid and singleminded in ventrolateral regions, which become neurogenic ectoderm (Ip et al. 1992b;Kasai et al. 1992). Low concentrations of Dorsal (or no Dorsal) result in the expression of decapentaplegic (dpp), zerknfillt (zen), and tolloid (tld) in dorsal and dorsolateral regions, which dif2Present address:

The Drosophila melanogaster decapentaplegic (dpp) gene encodes a transforming growth factor ␤-related cell signaling molecule that plays a critical role in dorsal/ventral pattern formation. The dpp expression pattern in the Drosophila embryo is dynamic, consisting of three phases. Phase I, in which dpp is expressed in a broad dorsal domain, depends on elements in the dpp second intron that interact with the Dorsal transcription factor to repress transcription ventrally. In contrast, phases II and III, in which dpp is expressed first in broad longitudinal stripes (phase II) and subsequently in narrow longitudinal stripes (phase III), depend on multiple independent elements in the dpp 5-flanking region. Several aspects of the normal dpp expression pattern appear to depend on the unique properties of the dpp core promoter. For example, this core promoter (extending from ؊22 to ؉6) is able to direct a phase II expression pattern in the absence of additional upstream or downstream regulatory elements. In addition, a ventral-specific enhancer in the dpp 5-flanking region that binds the Dorsal factor activates the heterologous hsp70 core promoter but not the dpp core promoter. Thus, the dpp core promoter region may contribute to spatially regulated transcription both by interacting directly with spatially restricted activators and by modifying the activity of proteins bound to enhancer elements.The transforming growth factor ␤ gene superfamily encodes a large set of cell signaling proteins with diverse roles in development, cell growth, and differentiation (21). These factors are synthesized as long precursors, which are processed to generate ϳ100-amino-acid polypeptide factors. The signals generated by these secreted factors are transmitted by membrane-bound receptor serine/threonine kinases (22). Because several members of the transforming growth factor ␤ superfamily contribute to spatially regulated developmental phenomena, a full understanding of the processes that are controlled by these factors will require an understanding of the mechanisms that control their spatially restricted synthesis and release.The product of the decapentaplegic (dpp) gene is one of several known transforming growth factor ␤-related factors in Drosophila melanogaster (23). Among its many roles in both embryonic and larval development, Dpp acts as a morphogen that helps to determine cell fate on the dorsal side of the embryo (8). The expression pattern of dpp in the developing embryo is controlled at the level of transcription and is extremely dynamic, consisting of three distinct phases (30). In phase I expression, which is first detected in the syncytial blastoderm (ϳ1.5 h postfertilization), dpp is transcribed in the dorsal 40% of the embryo, a region that includes the anlagen of the amnioserosa and the dorsal epidermis ( Fig. 1A and D). Phase I expression is dependent upon the maternally encoded Dorsal morphogen, which binds to elements in the dpp second intron to repress dpp expression in the ventral 60% of the blastoderm embryo (13)....

Stimulation of the type 1 IL-1R (IL-1R1) and the IL-18R by their cognate ligands induces recruitment of the IL-1R-associated kinase (IRAK). Activation of IRAK leads in turn to nuclear translocation of NF-κB, which directs expression of innate and adaptive immune response genes. To study IRAK function in cytokine signaling, we generated cells and mice lacking the IRAK protein. IRAK-deficient fibroblasts show diminished activation of NF-κB when stimulated with IL-1. Immune effector cells without IRAK exhibit a defective IFN-γ response to costimulation with IL-18. Furthermore, mice lacking the Irak gene demonstrate an attenuated response to injected IL-1. Deletion of Irak, however, does not affect the ability of mice to develop delayed-type hypersensitivity or clear infection with the intracellular parasite, Listeria monocytogenes. These results demonstrate that although IRAK participates in IL-1 and IL-18 signal transduction, residual cytokine responsiveness operates through an IRAK-independent pathway.