There are several known "consensus" sequences (sometimes called "motifs") associated with promoters which appear to be common among species which are necessary for expression of eukaryotic genes. These are binding sites for specific proteins necessary for DNA transcription. Examples include:
1. Zinc fingers - amino acid groups stabilized
by four cystine or cystine and histidine
residues associated with a zinc atom.
2. Helix-turn-helix - two amino acid a helices oriented at right angles to one another.
3. Leucine zippers
- found in oncoproteins, composed of multiple leucine residues
forming an a helice having side chains which project in a line from one side of the
protein molecule. These interdigitate like teeth of a zipper with another protein,
forming a stable dimer, which allows the proper alignment of protein on the DNA
Though sequences of promoter elements vary from one gene to another, they are found in most eukaryotic promoters. Each is named for its distinct nucleotide sequence and referred to as a "box":
1. The TATA
box is found 25-30 nucleotides upstream from
the transcription start
codon, and is necessary for the positioning of RNA polymerase in the initiation
2. The CAT
box (sequence CCAAT) is 70-80 nucleotides
upstream from the
transcription start sequence, and is the binding site for the CTF protein, which
is necessary for the intiation of adenovirus DNA replication. Not all promoters
contain this sequence, so the CTF protein must specifically regulate only genes
containing the CCAAT sequence.
3. The GC box
contains multiple guanine/cytosine-rich regions found in the promoters
of the so-called "housekeeping" genes which code for proteins essential to proper
metabolic functioning (constitutive proteins which are continuously synthesized).
Such alternating bands of purines and pyrimindines can flip from the normal
right-handed B-DNA helix to the left-handed Z-DNA helix, which is easily
recognized as a target for a DNA-binding protein.