The promoter activates transcription of a gene.  It is located at the 5' end of a gene, adjacent to the structural region (which codes for protein), and extends ~150-300 nucleotides upstream of the beginning of the transcription site.

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
     of transcription.

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.