Enhancers: These are DNA sequences that bind transcription factors and promote the transcription of nearby genes. For example, the enhancer in the immunoglobulin heavy-chain locus is responsible for the high level of expression of these genes in B cells.
Promoters: These are DNA sequences that bind RNA polymerase and initiate the transcription of genes. For example, the promoter in the human growth hormone gene is responsible for the expression of this gene in the pituitary gland.
Silencers: These are DNA sequences that bind transcription factors and repress the transcription of nearby genes. For example, the silencer in the human β-globin gene is responsible for the repression of this gene in non-erythroid cells.
Trans-acting sequences:
Transcription factors: These are proteins that bind to cis-acting sequences and regulate the transcription of genes. For example, the transcription factor Oct-1 binds to the octamer motif in the immunoglobulin heavy-chain enhancer and promotes the transcription of these genes in B cells.
RNA polymerase: This is a protein complex that binds to promoters and transcribes genes into RNA.
Other regulatory proteins: These are proteins that bind to cis-acting sequences and regulate the transcription of genes in various ways. For example, the protein CTCF binds to the CCCTC motif in the human β-globin gene and helps to organize the chromatin structure of this region, which is necessary for the repression of the gene in non-erythroid cells.