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Fig. 2. Strategies employed to interpret graded signals. (A)
Binding-site affinity. The number and affinity of transcription factor binding
sites determine threshold responses. Low amounts of transcriptional effector
are sufficient to bind to and activate transcription from high-affinity
binding sites; lower-affinity binding sites require larger amounts of
transcriptional effector. (B) Combinatorial inputs. The integration of
multiple positive and/or negative inputs with the transcriptional effector of
the morphogen establishes a threshold response. Other regulatory elements (X)
can also determine the response of a target gene. (C) Feed-forward
loop. A regulatory circuit in which the transcriptional effector activated by
the morphogen controls the expression of a second regulator (Y); the
combination of the two regulate the transcription of a target gene. (D)
Positive feedback. A gene (X) induced by the morphogen autoregulates to
enhance its own expression. (E) Cross repression. Repressive
interactions between morphogen-regulated genes (X and Y) establish discrete
changes in gene expression. Repressive interactions can be asymmetric (for
example ventral dominance in the Drosophila neurectoderm) or
symmetric, resulting in reciprocal cross repression (for example in the
vertebrate neural tube). (F) Reciprocal repressor gradient. The
transcriptional effector sets up an inverse transcriptional repressor gradient
that is interpreted by target genes. The ratio of repressor (R) to activator
defines the threshold response of target genes, depending on the binding sites
present in the enhancer.
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