Question

What properties demonstrate the lac repressor to be a protein? Describe the evidence that it indeed serves as a repressor within the operon system.

Short answer

Answer: The lac repressor has several properties that support its classification as a protein, including its structure (composed of polypeptide chains), molecular weight (around 150 kDa), amino acid composition (string of amino acids forming its primary structure), and specific binding activity towards the operator region of the lac operon. The evidence for its role as a repressor within the operon system includes binding to the operator region, inhibition of transcription, inducer-mediated release, and genetic evidence from loss-of-function or gain-of-function mutations.

Step-by-step solution

Properties of the lac repressor

The lac repressor is a protein with several properties that support its classification as a protein. These properties include: 1. Its structure: The lac repressor is composed of polypeptide chains, which are characteristic of proteins. 2. Molecular weight: The lac repressor has a molecular weight of around 150 kDa, which is consistent with the size of proteins. 3. Amino acid composition: Like other proteins, the lac repressor is composed of a string of amino acids, which form its primary structure. 4. Specific activity: The lac repressor displays specific binding activity towards the operator region of the lac operon, which implies a high degree of molecular specificity, a characteristic feature of proteins.

Evidence for lac repressor's role as a repressor in the operon system

The evidence for the role of the lac repressor as a repressor within the operon system includes: 1. Binding to the operator region: The lac repressor binds to the operator region of the lac operon in the absence of lactose, which demonstrates its role in controlling gene expression. 2. Inhibition of transcription: The binding of the lac repressor to the operator region physically blocks the access of RNA polymerase to the promoter region, thereby repressing the transcription of the lac operon genes. 3. Inducer-mediated release: The presence of an inducer molecule, such as allolactose or isopropyl-beta-D-thiogalactopyranoside (IPTG), leads to the release of the lac repressor from the operator region, allowing transcription to proceed. This conditional repression is evidence of the lac repressor's role as a repressor. 4. Genetic evidence: Mutations in the lac repressor can lead to a loss of repression (loss-of-function mutations) or constitutive repression (gain-of-function mutations), further substantiating its role as a repressor in the operon system.