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

The properties that demonstrate that the lac repressor is a protein include its structure, its ability to form complexes with specific DNA sequences and molecules, and its role in biological processes. Proteins are made up of amino acids and have a specific three-dimensional structure, they also participate in various biological processes such as catalysis, signaling, or molecular recognition.

The lac repressor has a specific three-dimensional structure, with four identical subunits made up of amino acids. It also specifically binds to the operator region of the lac operon. Properties such as the specific binding to a DNA sequence, as well as its role in gene regulation demonstrate key protein properties. In addition, the lac repressor is able to bind the inducer molecule, allolactose, which causes a conformational change and dissociation from the operator sequence, another typical property of proteins.

The lac repressor functions as a repressor within the operon system by binding to the operator region of the lac operon when lactose is absent in the cell. By binding to the operator region, it blocks the activity of RNA polymerase, preventing it from transcribing the downstream lac genes, and hence inhibiting the production of lactose metabolizing enzymes.

Key evidence for lac repressor's role in the operon system are: 1. Genetic mutations in the lac repressor gene (lacI) result in either a constitutive phenotype, where the lac genes are expressed regardless of lactose presence/absence (loss of function mutations), or a non-inducible phenotype, where the repressor is unable to respond to lactose (gain of function mutations). 2. The binding of the repressor to the operator sequence has been confirmed by electrophoretic mobility shift assays. This assay allows visualization of how DNA-protein complexes migrate through a gel compared to unbound DNA. When the lac repressor is present and bound to the operator, the DNA has a reduced mobility compared to the unbound state. 3. X-ray crystallography and other biophysical techniques have revealed the structure of the lac repressor, the manner in which it binds DNA, and the conformational changes induced by allolactose.