Question

Describe the difference between cis-acting and trans-acting elements. Also list whether the following elements are cis- or trans-acting:

A) Beta-galactosidase
B) Operator site
C) LacI repressor protein

D) Lactose
E) Permease
F) Promoter site

Answer 1

Cis-acting elements are regions of non-coding DNA and a trans-acting element is usually a DNA sequence that contains a gene.

Cis-acting elements do not directly code for protein or RNA. But trans-acting element codes for a protein that will be used in the regulation of another target gene.

Cis-acting elements regulate the transcription of nearby genes. But trans-acting elements may modify (or regulate) the expression of distant genes.

Cis-acting elements are present on the same molecule of DNA as the gene they regulate, whereas tans-acting elements can regulate genes far from the gene from which they were transcribed.

Cis-acting elements work through an intramolecular interaction between different parts of the same molecule whereas trans-acting elements work through an intermolecular interaction between two different molecules.

A) Beta b-galactosidase is responsible for hydrolysis of disaccharide lactose and is encoded by lac z gene.

B) The operator site in the lac operon is cis-acting regulatory sequence. This DNA sequence is bound by the lac repressor thus preventing transcription of the adjacent genes on the same DNA molecule. The operator itself does not code for any protein or RNA.

C) LacI repressor protein is a DNA-binding protein which inhibits the expression of genes coding for proteins involved in the metabolism of lactose in bacteria. They modify the expression of genes distant from the gene. So it can be considered as a trans-acting element.

D) Lactose is the substrate for the enzyme Beta b-galactosidase and it regulates switching on and off of the operon.

E) Permease increases permeability of the cell to Beta b-galactosidase and is encoded by lac y gene.

F) Promoter is the binding site for RNA polymerase. They are present near the target sequence. Hence it is a Cis-acting element.