Question

The locations of numerous lacI⁻ and lacIˢ mutations have been determined within the DNA sequence of the lacI gene. Among these, lacI⁻ mutations were found to occur in the 5′-upstream region of the gene, while lacIˢ mutations were found to occur farther downstream in the gene. Are the locations of the two types of mutations within the gene consistent with what is known about the function of the repressor that is the product of the lacI gene?

Accepted Answer

Step 1: Understand the function of the lacI gene product. The lacI gene encodes the lac repressor protein, which regulates the lac operon by binding to the operator region and preventing transcription in the absence of an inducer. Step 2: Recognize the difference between lacI⁻ and lacIˢ mutations. lacI⁻ mutations typically result in a nonfunctional repressor that cannot bind DNA, while lacIˢ mutations produce a super-repressor that binds DNA but cannot be inactivated by the inducer. Step 3: Consider the location of lacI⁻ mutations in the 5′-upstream region. This region often includes the promoter and DNA-binding domain coding sequences, so mutations here can disrupt the repressor's ability to bind the operator, consistent with loss of function. Step 4: Consider the location of lacIˢ mutations farther downstream in the gene. These mutations likely affect the inducer-binding domain of the repressor, preventing the repressor from responding to the inducer and remaining bound to DNA, consistent with the super-repressor phenotype. Step 5: Conclude that the spatial distribution of lacI⁻ and lacIˢ mutations within the gene aligns with the functional domains of the lac repressor protein, supporting the known molecular mechanism of repression and induction.

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Concetti chiave

lacI Gene and Its Product

Types of lacI Mutations: lacI⁻ and lacIˢ

Mutation Location and Functional Domains