Ch. 12 - Regulation of Gene Expression in Bacteria and Bacteriophage

Sanders - Genetic Analysis: An Integrated Approach 3rd Edition

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Sanders 3rd Edition

Ch. 12 - Regulation of Gene Expression in Bacteria and Bacteriophage

Problem 30b

Chapter 12, Problem 30b

For an E. coli strain with the lac operon genotype I⁺ P⁺ O⁺ Z⁺ Y⁺, identify the level of transcription of the operon genes in each growth medium listed. Specify transcription as 'none,' 'basal,' or 'activated' for each medium, and provide an explanation to justify your answer.
Growth medium contains glucose but no lactose.

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Understand the lac operon system: The lac operon in E. coli is a set of genes (Z and Y) involved in lactose metabolism. It is regulated by the presence or absence of lactose and glucose. The key components are: (1) the repressor protein (I), (2) the promoter (P), (3) the operator (O), and (4) the structural genes (Z and Y).

Analyze the genotype I⁺ P⁺ O⁺ Z⁺ Y⁺: This genotype indicates that all components of the lac operon are functional. The repressor protein (I⁺) can bind to the operator (O⁺) to block transcription in the absence of lactose. The promoter (P⁺) is functional, allowing RNA polymerase to bind and initiate transcription when the operon is activated.

Consider the growth medium: The medium contains glucose but no lactose. Glucose represses the lac operon through catabolite repression, a process mediated by low levels of cyclic AMP (cAMP). Low cAMP levels prevent the formation of the cAMP-CAP complex, which is required for activated transcription of the lac operon.

Determine the effect of no lactose: In the absence of lactose, the repressor protein (I⁺) binds to the operator (O⁺), physically blocking RNA polymerase from transcribing the Z and Y genes. This results in no transcription of the operon.

Conclude the transcription level: Since glucose is present (causing catabolite repression) and lactose is absent (allowing the repressor to block transcription), the transcription level of the lac operon genes is 'none.'

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Lac Operon Structure

The lac operon in E. coli consists of several key components: the promoter (P), operator (O), and structural genes (Z and Y) that code for enzymes involved in lactose metabolism. The I gene produces a repressor that binds to the operator, preventing transcription when lactose is absent. Understanding this structure is essential for analyzing how the operon responds to different growth media.

Glucose Effect

In the presence of glucose, E. coli prefers to utilize it over lactose due to catabolite repression. This mechanism involves the inhibition of the lac operon by the cAMP-CAP complex, which is not formed when glucose is abundant. Therefore, when glucose is present, the transcription of the lac operon is significantly reduced, leading to a low level of expression of the operon genes.

Transcription Levels

Transcription levels can be categorized as 'none,' 'basal,' or 'activated.' 'None' indicates no transcription occurs, 'basal' refers to minimal transcription under certain conditions, and 'activated' signifies high transcription levels due to the presence of inducers. In the context of the lac operon, understanding these levels helps determine how the operon responds to the absence of lactose and the presence of glucose.

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The function of tRNA synthetases is to attach amino acids to tRNAs. Suppose the tRNA synthetase responsible for attaching tryptophan to tRNA is mutated in a bacterial strain, with the result that the tRNA synthetase functions at about 15% of the efficiency of the wild-type tRNA synthetase.

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The bacterial insertion sequence IS10 uses antisense RNA to regulate translation of the mRNA that produces the enzyme transposase, which is required for insertion sequence transposition. Transcription of the antisense RNA gene is controlled by P_OUT, which is more than 10 times more efficient at transcription than the P_IN promoter, which controls transposase gene transcription.

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Textbook Question

For an E. coli strain with the lac operon genotype I⁺ P⁺ O⁺ Z⁺ Y⁺, identify the level of transcription of the operon genes in each growth medium listed. Specify transcription as 'none,' 'basal,' or 'activated' for each medium, and provide an explanation to justify your answer.

Growth medium contains lactose but no glucose.

Textbook Question

For an E. coli strain with the lac operon genotype I⁺ P⁺ O⁺ Z⁺ Y⁺, identify the level of transcription of the operon genes in each growth medium listed. Specify transcription as 'none,' 'basal,' or 'activated' for each medium, and provide an explanation to justify your answer.

Growth medium contains lactose and glucose.

Textbook Question

If a mutation reduced the transcriptional efficiency of P_OUT so as to be equal to that of P_IN, what is the likely effect on the transposition of IS10?

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