Final answer:
The Leucine zipper is the DNA binding domain that is a dimer held together by hydrophobic interactions involving side chains of amino acids
Step-by-step explanation:
The DNA binding domain that is a dimer held together by hydrophobic interactions involving the side chains of amino acids is the Leucine zipper. This domain has a characteristic dimerization interface where leucine residues are positioned every seventh position in an α-helix structure, and the leucines interlock with those on a parallel α-helix of another monomer, forming a zipper-like structure. These leucine residues are key to the hydrophobic interactions that stabilize the dimerization of the domain. Unlike the leucine zipper, the helix-turn-helix, zinc finger, and homeodomain do not rely as heavily on hydrophobic interactions between side chains to dimerize.
The leucine zipper is a motif used by certain transcription factors to bind to DNA, and it functions by promoting dimerization, which increases the protein's DNA-binding affinity and specificity. This is an essential mechanism for the regulation of gene expression.