<P> Originally discovered in bacteria, the helix - turn - helix motif is commonly found in repressor proteins and is about 20 amino acids long . In eukaryotes, the homeodomain comprises 2 helices, one of which recognizes the DNA (aka recognition helix). They are common in proteins that regulate developmental processes (PROSITE HTH). </P> <P> The zinc finger domain is generally between 23 and 28 amino acids long and is stabilized by coordinating zinc ions with regularly spaced zinc - coordinating residues (either histidines or cysteines). The most common class of zinc finger (Cys2His2) coordinates a single zinc ion and consists of a recognition helix and a 2 - strand beta - sheet . In transcription factors these domains are often found in arrays (usually separated by short linker sequences) and adjacent fingers are spaced at 3 basepair intervals when bound to DNA . </P> <P> The basic leucine zipper (bZIP) domain contains an alpha helix with a leucine at every 7th amino acid . If two such helices find one another, the leucines can interact as the teeth in a zipper, allowing dimerization of two proteins . When binding to the DNA, basic amino acid residues bind to the sugar - phosphate backbone while the helices sit in the major grooves . It regulates gene expression . </P> <P> Consisting of about 110 amino acids, the winged helix (WH) domain has four helices and a two - strand beta - sheet . </P>

Where do gene regulators bind on the dna double helix