Cis-regulatory element

A cis-regulatory element or cis-element is a region of DNA or RNA that regulates the expression of genes located on that same molecule of DNA (often a chromosome). This term is constructed from the Latin word cis, which means "on the same side as". These cis-regulatory elements are often binding sites for one or more trans-acting factors. A cis-element may be located 5' to the coding sequence of the gene it controls (in the promoter region or further upstream), in an intron, or 3' to the gene's coding sequence, either in the untranslated or untranscribed region.

An example of a cis-acting regulatory sequence is the operator in the lac operon. This DNA sequence is bound by the lac repressor, which, in turn, prevents transcription of the adjacent genes on the same DNA molecule. The lac operator is, thus, considered to "act in cis" on the regulation of the nearby genes. The operator itself does not code for any protein or RNA.

In contrast, trans-regulatory elements are diffusible factors, usually proteins, that may modify the expression of genes distant from the gene that was originally transcribed to create them. For example, a transcription factor which regulates a gene on chromosome 6 might itself have been transcribed from a gene on chromosome 11. This term is constructed from the Latin root trans, which means "across from".

To summarize, cis-regulatory elements are present on the same molecule of DNA as the gene they regulate whereas trans-regulatory elements can regulate genes distant from the gene from which they were transcribed.