Question

Since the initiation of eukaryotic transcription involves the interaction of a multitude of transcription factors, there must be regulation of these. Estrogen normally binds to its nuclear receptor and the complex binds to the sterol response element to regulate transcription. Tamoxifen, a drug used to treat breast cancer, competes for the estrogen receptor and reduces transcription of the genes it regulates. Another type of alteration is seen in Holt-Oram syndrome in which there is a mutation in a gene for a transcription factor \((\mathrm{Tbx}),\) leading to defects in formation of the heart. Transcription factors are frequently sequence-specific binding proteins and most likely have one of several specific structural motifs. The helix-turn- helix motif A. coordinates zinc between cysteines and histidines. B. joins two proteins via hydrophobic interactions between leucines. C. forms dimers held together by interaction of a helix on each monomer. D. has one helix that recognizes and binds in the major groove of DNA while hydrophobic interactions with a second helix stabilizes the structure. E. is the only motif that binds in the major groove of DNA.

Short answer

A. coordinates zinc between cysteines and histidines B. joins two proteins via hydrophobic interactions between leucines C. forms dimers held together by interaction of a helix on each monomer D. has one helix that recognizes and binds in the major groove of DNA while hydrophobic interactions with a second helix stabilizes the structure E. is the only motif that binds in the major groove of DNA Answer: D. has one helix that recognizes and binds in the major groove of DNA while hydrophobic interactions with a second helix stabilizes the structure

Step-by-step solution

Understand the helix-turn-helix motif

The first step in solving this problem is learning about the helix-turn-helix (HTH) motif. It is a protein structural motif found in sequence-specific DNA-binding proteins. This motif is characterized by two alpha helices connected by a short strand of amino acids, and one of the helices binds to the major groove of DNA.

Review answer choices

Next, study each answer choice carefully and compare them to the facts about the HTH motif: A. coordinates zinc between cysteines and histidines -> This description refers to the zinc finger motif, not the HTH motif. B. joins two proteins via hydrophobic interactions between leucines -> This description refers to the leucine zipper motif, not the HTH motif. C. forms dimers held together by interaction of a helix on each monomer -> This description refers to coiled-coil dimerization motifs, not the HTH motif. D. has one helix that recognizes and binds in the major groove of DNA while hydrophobic interactions with a second helix stabilizes the structure -> This description correctly describes the HTH motif, where one helix interacts with the DNA major groove, and the other helix provides structural stability. E. is the only motif that binds in the major groove of DNA -> This statement is false, as there are other DNA-binding motifs that also interact with the major groove of DNA (e.g., zinc finger and leucine zipper motifs).

Select the correct answer

The correct description of the helix-turn-helix motif is found in option D: "has one helix that recognizes and binds in the major groove of DNA while hydrophobic interactions with a second helix stabilizes the structure."