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Chapter 15: Problem 8

Describe the experimental rationale that allowed the lac repressor to be isolated.

Short Answer

Expert verified
Answer: The isolation of the lac repressor protein was a crucial step in understanding gene regulation. By isolating the lac repressor, scientists were able to investigate its molecular structure, its binding to the operator region, and its conformational changes in the presence of an inducer molecule. This work provided foundational knowledge for understanding the mechanisms of gene expression regulation in all organisms, not just bacteria.

Step by step solution

01

Understand the lac operon system

The lac operon is a group of genes responsible for the transport and metabolism of lactose in bacteria like E. coli. The operon consists of three structural genes (lacZ, lacY, and lacA) and regulatory elements, including the operator region (lacO) and the promoter region (lacP). The lac repressor protein binds to the operator region, physically blocking transcription of the structural genes when lactose is not present in the environment, preventing wasteful energy expenditure.
02

Identify the role of the lac repressor

The lac repressor is a critical component of the lac operon's regulatory system. It acts as a molecular switch, turning the operon on and off by binding to the operator region in the absence of an inducer molecule (such as allolactose). This binding prevents RNA polymerase from transcribing the genes encoding for the enzymes necessary to break down lactose within the cell.
03

Employ mutants in the isolation process

In the 1960s, researchers François Jacob and Jacques Monod used bacterial strains with mutations in the lac operon to investigate the regulatory mechanisms. They isolated mutants that expressed the lac genes constitutively, regardless of the presence of lactose. By comparing these mutants to the wild-type strains, they could infer the function of the protein responsible for regulating the lac operon. This led to the identification of the lac repressor.
04

Use of affinity chromatography

After establishing the existence and role of the lac repressor, scientists needed to purify and isolate it from the cell to study it further. Walter Gilbert and Benno Müller-Hill developed a technique called affinity chromatography to isolate the protein. They immobilized a specific DNA sequence, called the lac operator region, on a solid support. The lac repressor, when present in a cell extract, would specifically bind to the attached operator sequence. By washing away the other proteins and then eluting the lac repressor, they successfully isolated the protein.
05

Significance of the isolated lac repressor

The isolation of the lac repressor was a crucial step in understanding gene regulation. The purified lac repressor facilitated further investigation into its molecular structure, how it binds to the operator region, and how it changes conformation in the presence of an inducer molecule. Ultimately, this work on the lac repressor laid the foundation for understanding how gene expression is regulated and controlled in all organisms, not just bacteria.

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Most popular questions from this chapter

Describe the role of attenuation in the regulation of tryptophan biosynthesis.

Predict the effect on the inducibility of the lac operon of a mutation that disrupts the function of (a) the crp gene, which encodes the CAP protein, and (b) the CAP-binding site within the promoter.

A bacterial operon is responsible for the production of the biosynthetic enzymes needed to make the hypothetical amino acid tisophane (tis). The operon is regulated by a separate gene, \(R\) The deletion of \(R\) causes the loss of enzyme synthesis. In the wildtype condition, when tis is present, no enzymes are made; in the absence of tis, the enzymes are made. Mutations in the operator gene \(\left(O^{-}\right)\) result in repression regardless of the presence of tis. Is the operon under positive or negative control? Propose a model for (a) repression of the genes in the presence of tis in wild-type cells and (b) the mutations.

Review the Chapter Concepts list on p. \(285 .\) These all relate to the regulation of gene expression in bacteria. Write a brief essay that discusses why you think regulatory systems evolved in bacteria (i.e., what advantages do regulatory systems provide to these organisms?), and, in the context of regulation, discuss why genes related to common functions are found together in operons.

Why is the CRISPR-Cas system of bacteria considered an adaptive immunity rather than an innate immunity?
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