Question

Name one type of dna bonding domain of eukaryotic activators and describe its structure

Answer 1

Transcription activators in eukaryotes contain at least one DNA-binding domain (DBD) and transcription-activation domain (TAD). The structural motifs found in transcription factor proteins are of the following types -

• Helix-turn-helix motif
• Helix-loop-helix motif
• Zinc finger motif
• Leucine zipper motif

Zinc- coordinated DNA-binding motifs are called zinc finger motifs. It is composed of one ? helix and two anti-parallel ?-sheet structures. A zinc (Zn2+) metal ion stablizes the structure as it is essential for correct folding and DNA-binding.

One of the most widely occuring zinc finger motif in eukaryotes is the Cys₂ - His₂ (C₂ - H₂) zinc finger. It regulates the transcription of genes for 5S rRNA and always occurs in clusters. The length of each finger is nearly 30 amino acids long and is characterized by pairs of Cys and His. The consensus sequence of a single finger is -

-Cys-X_2-4 -Cys-X₃-Phe-X₅-Leu-X₂-His-X₃-His-

The zinc ion is coordinated tetrahedrally by the Cys and His residues. The -helix of each finger lies in the major groove and contacts three base pairs.