Question

Please emphasize if the following are cis-acting, trans-acting or both on the lac operon.

The lac repressor Protein:
The Operator sequence:
The Operon sequence:
RNA Polymerase II

Answer 1

There are several factors that are involved in gene regulation. Cis-acting and trans-acting factors/ elements are one of these factors which regulate the gene expression at the transcription level. Cis-acting elements are non-coding DNA regions that regulate the expression of their adjacent genes. Trans-acting factors are regulatory proteins that can regulate the expression of genes that are far located or even located on other chromosomes. Cis-acting elements do not code for proteins whereas trans-acting elements code for RNA/proteins. Transcription factors are an example of trans-acting elements whereas promoters are example of cis-acting elements.

The lac repressor protein: It is a trans-acting element because if it is present, it is free and can bind to its binding sequence present in the genome. It's not necessary for a repressor protein to regulate its adjacent DNA sequence product.

The operator sequence: It is a cis-acting element. The operator sequence is a DNA region of an operon that is present adjacent to structural genes and regulates their expression. It is a non-coding DNA and regulates only adjacent DNA as changes in operator sequence affect only adjacent gene expression.

The operon sequence: The operon sequence includes both cis-acting and trans-acting elements. The promoter, the operator are cis-acting elements whereas regulatory sequence (repressor protein in case of lac operon) is a trans-acting element.

RNA polymerase II: It belongs to trans-acting factors. It binds to the promoter region of the coding sequences all over the genome. Although in true sense it is an enzyme that catalyzes the transcription process whereas cis and trans-acting elements help RNA polymerase and regulate transcription.