The Helix TurnHelix Motif

The first motif described—and the one studied most extensively—is the helix-turn-helix. Analysis of the three-dimensional structure of the X Cro transcription regulator has revealed that each monomer consists of three antiparallel P sheets and three a helices (Figure 39-13). The dimer forms by association of the antiparallel P3 sheets. The a3 helices form the DNA recognition surface, and the rest of the molecule appears to be involved in stabilizing these structures. The average diameter of an a helix is 1.2 nm, which is the approximate width of the major groove in the B form of DNA. The DNA recognition domain of each Cro monomer interacts with 5 bp and the dimer binding sites span 3.4 nm, allowing fit into successive half turns of the major groove on the same surface (Figure 39-13). X-ray analyses of the X cl repressor, CAP (the cAMP receptor

Figure 39-13. A schematic representation of the three-dimensional structure of Cro protein and its binding to DNA by its helix-turn-helix motif. The Cro monomer consists of three antiparallel p sheets (P1-P3) and three a-helices (a,-a3). The helix-turn-helix motif is formed because the a3 and a2 helices are held at about 90 degrees to each other by a turn of four amino acids. The a3 helix of Cro is the DNA recognition surface (shaded). Two monomers associate through the antiparallel P3 sheets to form a dimer that has a twofold axis of symmetry (right). A Cro dimer binds to DNA through its a3 helices, each of which contacts about 5 bp on the same surface of the major groove. The distance between comparable points on the two DNA a-helices is 34 Á, which is the distance required for one complete turn of the double helix. (Courtesy of B Mathews.)

Figure 39-13. A schematic representation of the three-dimensional structure of Cro protein and its binding to DNA by its helix-turn-helix motif. The Cro monomer consists of three antiparallel p sheets (P1-P3) and three a-helices (a,-a3). The helix-turn-helix motif is formed because the a3 and a2 helices are held at about 90 degrees to each other by a turn of four amino acids. The a3 helix of Cro is the DNA recognition surface (shaded). Two monomers associate through the antiparallel P3 sheets to form a dimer that has a twofold axis of symmetry (right). A Cro dimer binds to DNA through its a3 helices, each of which contacts about 5 bp on the same surface of the major groove. The distance between comparable points on the two DNA a-helices is 34 Á, which is the distance required for one complete turn of the double helix. (Courtesy of B Mathews.)

protein of E colt), tryptophan repressor, and phage 434 repressor all also display this dimeric helix-turn-helix structure that is present in eukaryotic DNA proteins as well (see Table 39-3).

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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