Several Motifs Mediate The Binding Of Regulatory Proteins To

The specificity involved in the control of transcription requires that regulatory proteins bind with high affinity to the correct region of DNA. Three unique motifs— the helix-turn-helix, the zinc finger, and the leucine

REPORTER GENE CONSTRUCTS WITH VARIABLE AMOUNTS OF 5'-FLANKING DNA

HORMONE-DEPENDENT TRANSCRIPTION INDUCTION

A

B

C

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o

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o

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o

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2000 1000 +1 Nucleotide position

W—I—I—I—I—I—IAH—I—I—■• HRE HRE HRE

Figure 39-11. Location of hormone response elements (HREs) A, B, and C using the reporter gene-transfection approach. A family of reporter genes, constructed as described in Figure 39-10, can be transfected individually into a recipient cell. By analyzing when certain hormone responses are lost in comparison to the 5' deletion, specific hormone-responsive elements can be located.

zipper—account for many of these specific proteinDNA interactions. Examples of proteins containing these motifs are given in Table 39-3.

Comparison of the binding activities of the proteins that contain these motifs leads to several important generalizations.

(1) Binding must be of high affinity to the specific site and of low affinity to other DNA.

(2) Small regions of the protein make direct contact with DNA; the rest of the protein, in addition to pro-

Table39-3. Examples of transcription regulatory proteins that contain the various binding motifs.

Binding Motif

Organism

Regulatory Protein

Phage

AcI, cro, and tryptophan and

434 repressors homeo box proteins Pit-1, Octl, Oct2

Drosophila

Xenopus

Serendipity, Hunchback TFIIIA

steroid receptor family, Sp1

Leucine zipper

C/EBP, fos, Jun, Fra-1, CRE binding protein, c-myc, n-myc, I-myc

Gene A

Gene B

Gene C

Figure 39-12. Combinations of DNA elements and proteins provide diversity in the response of a gene. Gene A is activated (the width of the arrow indicates the extent) by the combination of activators 1, 2, and 3 (probably with coactivators, as shown in Figure 37-10). Gene B is activated, in this case more effectively, by the combination of 1, 3, and 4; note that 4 does not contact DNA directly in this example. The activators could form a linear bridge that links the basal machinery to the promoter, or this could be accomplished by looping out of the DNA. In either case, the purpose is to direct the basal transcription machinery to the promoter. Gene C is inactivated by the combination of 1, 5, and 3; in this case, factor 5 is shown to preclude the essential binding of factor 2 to DNA, as occurs in example A. If activator 1 helps repressor 5 bind and if activator 1 binding requires a ligand (solid dot), it can be seen how the ligand could activate one gene in a cell (gene A) and repress another (gene C).

viding the trans-activation domains, may be involved in the dimerization of monomers of the binding protein, may provide a contact surface for the formation of het-erodimers, may provide one or more ligand-binding sites, or may provide surfaces for interaction with coac-tivators or corepressors.

(3) The protein-DNA interactions are maintained by hydrogen bonds and van der Waals forces.

(4) The motifs found in these proteins are unique; their presence in a protein of unknown function suggests that the protein may bind to DNA.

(5) Proteins with the helix-turn-helix or leucine zipper motifs form symmetric dimers, and their respective DNA binding sites are symmetric palindromes. In proteins with the zinc finger motif, the binding site is repeated two to nine times. These features allow for cooperative interactions between binding sites and enhance the degree and affinity of binding.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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