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Boundary Creation by Different Combinations of Transcription Factors

In Section 15.3, we saw that maternally derived Bicoid plays a key role in initiating the transcription of gap genes (e.g., hunchback) in the anterior region of the early fly embryo; other maternal factors prevent the translation of hunchback mRNA in the posterior. Further specification of cell fates in Drosophila is controlled by transcription cascades in which one transcription factor activates a gene encoding another transcription factor, which in turn acts to promote the expression of a third transcription factor. Such a transcription cascade can generate a population of cells that may all look alike but differ at the transcriptional level.

Transcription cascades have both a temporal and a spatial dimension. At each step in a cascade, for instance, RNA poly-merase and ribosomes can take more than an hour to produce a protein, depending on the length of the corresponding gene. Spatial factors come into play when cells at different positions within an embryo synthesize different transcription factors. In this section, we continue the story of early patterning in Drosophila, which illustrates both the spatial and the temporal aspects of transcription cascades. The principles learned from the study of Drosophila development broadly apply to the creation of form and pattern in all organisms including plants, as discussed at the end of this section. Errors in the genes that control organization, boundary formation, and cell type are associated with many human diseases.

Drosophila Gap Genes Are Transcribed in Broad Bands of Cells and Regulate One Another

The rough outline of cell fates that is laid down in the syn-cytial fly embryo is refined into a system for precisely controlling the fates of individual cells. The discovery of the relevant regulators came from a genetic screen for mu tants with altered embryo body segments. The embryonic body segments go on to grow into the familiar striped pattern seen on any passing hornet. In addition to hunchback, four other gap genes—Krüppel, knirps, giant, and tailless—are transcribed in specific spatial domains, beginning about 2 hours after fertilization and just before cellularization of the embryo is complete (Figure 15-22a).

All the gap-gene proteins are transcription factors. Because these proteins are distributed in broad overlapping peaks (Figure 15-22b), each cell along the anterior/posterior axis contains a particular combination of gap-gene proteins that activates or represses specific genes within that cell. Indeed, something like a battle ensues, because some gap proteins repress the transcription of genes encoding other gap proteins. Although they have no known extracellular lig-ands, some gap proteins resemble nuclear receptors, which are intracellular proteins that bind lipophilic ligands (e.g., steroid hormones) capable of crossing the plasma membrane. Most ligand-nuclear receptor complexes function as transcription factors (see Figure 11-44). The sequence similarity between gap proteins and nuclear receptors suggests that gap genes may have evolved from genes whose transcription was controlled by signals that could cross membranes, such as the steroid hormones. The use of such signal-controlled genes, rather than transcription cascades, could explain how early cell-fate specification operates in animals that do not have a syncytial stage.

(a) Gap-gene proteins

(b) Hunchback and Krüppel

(c) Even-skipped and fushi tarazu

(a) Gap-gene proteins

Hunchback

Hunchback

Krüppel

(b) Hunchback and Krüppel

(c) Even-skipped and fushi tarazu

Krüppel

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