Boundary Creation by Different Combinations of Transcription Factors
■ Gradients of transcription factors, produced from maternal mRNAs in the early Drosophila embryo, control the patterned expression of embryonic genes, leading to segmentation of the embryo along the anteroposterior axis.
■ Target genes whose regulatory regions contain multiple enhancers are expressed preferentially in specific regions of the embryo, depending on the amounts and combinations of the transcription factors that control them (see Figure 15-23).
■ Early patterning events, utilizing maternal, gap, pair-rule, and segment-polarity genes, generate a unique pattern of transcription factors in different regions along the anteroposterior axis of Drosophila embryos (see Figure 15-24). These transcription factors are expressed transiently and play an essential role in establishing the domains in which different Hox genes are expressed.
■ Hox genes, which encode transcription factors, control the unique morphologic characteristics of different regions along the anteroposterior axis in most or all animals.
■ Misexpression of Hox genes causes homeotic transformations—the development of body parts in abnormal positions.
■ Hox expression patterns are sometimes maintained through positive autoregulatory loops, and through modulation of chromatin by proteins encoded by Polycomb-group and Trithorax-group genes.
■ Three classes of genes (A, B, and C) participate in specifying the identity of the four organs constituting a flower (see Figure 15-27). The patterns of expression of these genes, many of which encode transcription factors, are consistent with the ABC genetic model.
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