Final answer:
The leucine zipper is the DNA binding motif composed of three alpha helices, and it is a structure used by numerous transcription factors for DNA binding.
Step-by-step explanation:
The DNA binding motif composed of three alpha helices is known as the leucine zipper. This motif is characterized by regularly spaced leucine residues that facilitate dimerization, where two similar proteins come together, forming a stable structure. The leucine zipper is thus a structural motif used for DNA binding and is often found in transcription factors which play a critical role in gene expression.
In contrast, zinc fingers typically consist of a zinc ion coordinated by several amino acid residues, including cysteines and histidines, which stabilize a loop and an alpha helix that interacts with DNA. The helix-turn-helix motif is seen in many DNA-binding proteins and consists of two alpha helices joined by a short strand of amino acids that can recognize specific sequences of DNA. The homeodomain motif is usually found in proteins regulating pattern formation in developing embryos and has one helix that inserts into the major groove of DNA with an adjacent helix-turn-helix structure.