Tfa Tfb Tfc

Rb gene A now expressed in domain of TFd

Figure 8.1

Molecular mechanisms of c/s-regulatory DNA evolution

Four modes of c/s-regulatory element evolution are depicted. (a) De novo evolution of an element from nonfunctional sequences could occur through mutations (A,, A/,, and so on) that create binding sites for transcription factors (TF) in proximity to a transcription unit. (b) New elements may evolve by duplications of genes or of c/s-regulatory elements, followed by sequence divergence (left), or DNA rearrangements that move elements to new sites in the genome (right). (c) Modifications of existing c/s-regulatory elements can occur through the gain, loss, or change in affinity of sites for activators (TF) or repressors (R). (d) A special case of c/s-regulatory element modification is co-option of an existing element to function in a new domain through the gain of sites for specific factors (TFD).

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