Question

The locations of numerous \(\operatorname{lac} I\) and \(\operatorname{lac} I^{\mathrm{S}}\) mutations have been determined within the DNA sequence of the lacI gene. Among these, \(l a c I\) mutations were found to occur in the \(5^{\prime}\) -upstream region of the gene, while \(\operatorname{lac} I^{\mathrm{S}}\) 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?

Short answer

Explain. Answer: Yes, the locations of the lacI and lacIS mutations within the lacI gene are consistent with the function of the repressor that is the product of the lacI gene. The lacI mutations are found in the regulatory region, which can potentially affect the binding of the repressor and change the regulation of the lactose operon. In contrast, the lacIS mutations are found farther downstream, affecting the repressor's structure and function, resulting in a super-repressor.

Step-by-step solution

Function of the lacI gene

The lacI gene is responsible for producing the lac repressor, a protein that regulates the expression of genes involved in lactose metabolism in bacteria. The lac repressor binds to the promoter region of the lactose operon, preventing RNA polymerase from transcribing the genes involved in lactose metabolism when lactose is absent. When lactose is present, the repressor proteins bind to the lactose and release from the DNA, allowing the lactose metabolism genes to be transcribed and expressed.

Location of lacI mutations

The lacI mutations were found to occur in the 5'-upstream region of the gene. This region is critical for the regulation of gene expression since it contains the promoter and the operator region where the repressor protein binds to regulate transcription. Mutations in this region can potentially affect the binding of the repressor and, as a consequence, change the regulation of the lactose operon.

Location of lacIS mutations

The lacIS mutations were found to occur farther downstream in the gene. Since they are not located in the regulatory regions, these mutations likely affect the protein's structure and function. A lacIS mutation leads to a super-repressor, which is unable to respond to the presence of lactose and remains bound to the operator, preventing transcription even when lactose is available.

Consistency of mutation locations with repressor function

The locations of the two types of mutations within the gene are consistent with what is known about the function of the repressor produced by the lacI gene. The lacI mutations are found in the regulatory region, which can potentially affect the binding of the repressor and change the regulation of the lactose operon. On the other hand, the lacIS mutations are found farther downstream, affecting the repressor's structure and function, resulting in a super-repressor.