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First published online December 1, 2003
doi: 10.1242/10.1242/dev.00890

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Information display by transcriptional enhancers

Department of Biochemistry and Molecular Biology, Program in Genetics, Michigan State University, East Lansing, MI 48824-1319, USA

View larger version (64K): [in a new window]   Fig. 1. Simultaneous repression and activation from a compact regulatory element. (A) Knirps repression of adjacent Dorsal and Twist activators. Dorsal and Twist proteins, normally active in a broad (22-24 nuclei) ventral swathe of the blastoderm embryo, fail to activate a linked Hsp70 lacZ transgene in regions containing Knirps (kni) protein (arrow). (B) Giant repression of Dorsal and Twist. Repression is seen in anterior and posterior regions where the Giant (gt) repressor is expressed (arrows). (C,D) Gal4 activators, expressed in a narrower (18-20 nuclei) ventral swathe, are not inhibited by Knirps and Giant. (E) A composite element containing Dorsal, Twist and Gal4 activators exhibits repression of Dorsal and Twist by Knirps, while the narrower Gal4-driven expression pattern is unaffected. (F) A composite element with Dorsal, Twist and Gal4 activators, and Giant repressor, exhibits a similar complex expression pattern (arrows). (G) A similar pattern of selective repression of the Dorsal and Twist activators within the composite element used in F is seen when the activator Gal4 is driven throughout the embryo under the control of the nanos promoter (NGT40, Bloomington Stock no. 4442). In the central regions of the embryo more intense staining is visible, indicative of additive or synergistic gene activation by Dorsal, Twist and Gal4. In the regions of the embryo where the repressor Giant is expressed (arrows), the intensity of lacZ staining is the same as in the dorsal regions of the embryo where activation is driven by Gal4 alone. The difference in lacZ staining intensity between cells containing or lacking Giant or Knirps is due to a difference in intensity in each cell, not the number of cells stained. Patterns of gene expression were visualized in 2-4 hour embryos by in situ hybridization with digU-labeled antisense lacZ probes. Embryos are oriented anterior to left; ventrolateral views (A-E,G) and ventral view (F) are shown.

View larger version (49K): [in a new window]   Fig. 2. Compact regulatory element displays enhancer-like properties of distance and orientation independence. (A,D) The regulatory element shown in Fig. 1F was inserted in either orientation into a vector containing divergently transcribed white and transposase lacZ reporter genes. When situated at -265 bp, Dorsal/Twist (dl/twi) activators within the element drive expression of the white reporter gene. Repression by Giant (gt) is evident in anterior and posterior regions (arrows). (B,E) In the presence of Dorsal, Twist, and Gal4 activators, a composite pattern of gene regulation is seen as in Fig. 1F with inhibition of Dorsal/Twist and activation by Gal4. (C,F) A similar expression pattern is observed with the divergently transcribed transposase lacZ promoter, with repression by Giant of Dorsal/Twist and activation by Gal4. Embryos are oriented anterior to left; lateral views (A,D) and ventrolateral views (B,C,E,F) are shown.

View larger version (65K): [in a new window]   Fig. 3. Conversion of a multiple state element to a binary on/off switch. Two additional Giant (gt) binding sites were introduced at the 3' end of the Gal4 activator cluster. (A,B) As observed previously, Dorsal/Twist (dl/twi) activators are repressed in anterior and posterior regions of Giant expression (arrows). (C,D) In the presence of Dorsal/Twist and Gal4 activators, complete repression of transcription is observed in areas of Giant expression (arrows). Embryos are oriented anterior to the left. Lateral (A,C) and ventral (B,D) views are shown.

View larger version (28K): [in a new window]   Fig. 4. Enhanceosome versus Information Display enhancer models. (A) In the enhanceosome model, the enhancer serves as an information processing center, receiving inputs from multiple transcription factors that bind it. A highly structured complex or enhanceosome, creates a stereospecific interface for docking with and recruiting the basal transcription machinery. Here the enhancer serves as a molecular computer, resolves multiple inputs and provides a single output to the basal transcription machinery. With such an enhancer, the target gene would be activated only upon the assembly of a complex, providing a precise on/off binary transcriptional switch in response to the appropriate stimulus. Graded responses from such an element could be achieved by varying the stability of the entire complex, possibly in response to activator concentrations. (B,C) Information Display or "Billboard' enhancer. Rather than acting as a central processing unit, subelements can display contrasting information, which is then interpreted by basal transcription machinery. In this model, the basal machinery `samples' discrete regions of the enhancer each composed of a small number of transcription factor binding sites, either iteratively (B) or simultaneously (C). Successive/multiple interactions with the basal machinery, and the biochemical consequence of these interactions, would dictate the overall output of the enhancer.

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