Which of the following describes a trans-acting element?

A trans-acting element refers to a molecule, typically a protein, that can regulate the expression of genes located on different molecules of DNA. This means that the regulatory activity is not limited to the DNA sequence from which the molecule is derived; rather, it can influence the expression of target genes elsewhere in the genome.

In this context, the choice stating "a DNA sequence coding for a protein used for regulation" accurately captures the essence of trans-acting elements. These proteins, produced from the coding sequences, can bind to specific DNA sequences, like enhancers and promoters, to modulate gene expression. This capability to act on target genes that are not physically linked to it characterizes a trans-acting factor.

The other options do not adequately represent the concept of trans-acting elements. For instance, a segment of DNA coding for non-coding RNA typically functions in a more localized capacity or may have other roles, and a sequence that only acts on itself describes a cis-acting element, rather than a trans-acting one. Lastly, an enhancer located downstream of a promoter generally acts in a cis-acting manner, enhancing the expression of a nearby gene rather than trans-acting to influence genes that are located elsewhere.