Most signaling pathways operate through similar logic but have different proteins and signal transduction mechanisms. Signaling begins when membrane-bound receptors bind a ligand, leading to the release or activation of associated intracellular proteins. Receptor activation often leads to the modification of inactive transcription factors that are translocated to the cell nucleus, bind to c/s-regulatory DNA sequences or to DNA-binding proteins, and regulate the level of target gene transcription.
Because of the essential roles played by toolkit genes in the initial pattern of events in Droso-phila development, flies with mutations in toolkit genes typically fail to survive the early stages of the life cycle. Most of these genes also play critical roles later in development, however. Toolkit genes with multiple functions during development are termed pleiotropic. For instance, individual genes may act in different developmental events in the same tissue over the course of development, in different tissues at the same stage of development, or in different tissues over the entire course of development. The multiple roles played by individual genes can be unmasked by mutations that affect only a subset of a gene's functions, by conditional mutations (such as temperature-sensitive mutations), or by genetic techniques for investigating the requirements for gene function in subsets of cells at various stages of development.
Signaling pathway components are especially pleiotropic. For example, the Wingless pathway plays diverse roles in many tissues throughout Drosophila development. It is required early in the ectoderm to organize segment polarity; later in the embryonic ectoderm to direct the formation of the leg and wing imaginal discs; days later in the larval wing field to organize dorsoventral polarity, wing outgrowth, and sensory organ patterning; and finally to organize the polarity of the eye, leg, and other tissues. All components of the Wingless pathway are required in each of these settings, even though the ultimate regulatory and morphogenetic "output" of the pathway is different in each case. In Chapter 3, we will see how transcription factors and signaling pathways achieve their tissue-specific effects.
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