Trans-acting and cis-acting

From Wiki

Trans-acting means "acting from a different molecule". A trans-acting element is a DNA sequence that contains a gene. This gene codes for a protein (or a microRNA or other molecules) that will be used in the regulation of another target gene. The trans-acting element and the target gene can be on the same chromosome. But the activity of the trans-acting element is via the protein or RNA coded by the element.

Cis-acting means "acting from the same molecule". A cis-acting element is a DNA sequence that regulates the gene on the same chromosome via transcription factors or other trans-acting elements. An example 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 considers to "act in cis" on the regulation on the nearby gene because the operator does not code any protein or other molecule to regulate the nearby gene.