Lac Operon Genetics Practical Lab Report

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These strains are therefore phenotypically lac?. Genotypically, these strains could be lacZ? Or lacY?, since either mutation would result in an inability to metabolize lactose. Other mutations or combinations could also explain this phenotype, but are not being explored here. A lacZ? strain would not be able to produce a viable beta-galactosidase enzyme and therefore could not metabolize lactose and reduce the pH of the surrounding growth medium. The lacY gene produces a promiscuous permease transporter required for lactose entry into the bacterium, and in its absence beta-galactosidase has no substrate to hydrolize. Therefore, the genotype of the J. And K. strains are most likely [lacZ? Or lacY?], and lacI+.

The beta-galactosidase activity of the WT strain on nutrient agar plates with glucose or lactose, as assayed using MUG overlays, revealed the suppression of lac operon expression in the presence of glucose and its upregulation in the presence of lactose. This is consistent with glucose acting as a suppressor of lac operon expression.

In contrast, the lac operon is constitutively expressed in the H. strain in the presence of either glucose or lactose. This suggests that the mechanism responsible for suppression of the lac operon by glucose has been compromised. This is consistent with the H. strain having a genotype of lacZ+, lacY+, and lacI?.

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The beta-glactosidase activity of the J. And K. strains is the same whether in the presence of glucose or lactose and are therefore indifferent to the two conditions. The J. strain is capable of low-levels of beta-galactosidase activity in the presence of glucose and lactose, but the K. strain is not. Therefore the J. strain is lacZ+, which in turn suggests the sugars are unable to efficiently enter the bacterium. This suggests the genotype of the J. strain is lacZ+, lacY?, and lacI+ and the genotype of the K. strain is lacZ?, lacY+, and lacI+.

7. In your own words, explain the mechanism by which regulation of E. coli trp operon gene expression responds to levels of tryptophan in growth medium. What are the similarities and differences compared to lac operon regulation? (3 marks)

Lab Report on Lac Operon Genetics Practical 2. Assignment

The E. Coli trp operon contains the same regulatory components as the lac operon, with the difference that the gene encoding the repressor is not located near the operon and the operator element is located within the trp promoter. Another important difference is the trp operon encodes the genes necessary for tryptophan synthesis, rather than its metabolism.

The major regulatory difference is that the trp system is repressible, which means an effector molecule, in this case tryptophan, binds to the repressor and increases its activity. The result is that increasing tryptophan levels suppresses trp operon transcriptional activity via increased suppressor activity. This represents a negative feedback loop that regulates how much of the cell's energy is devoted to tryptophan production. In contrast, the lac system is an inducible system, such that the production of allolactose binds to and inhibits the lac repressor, thereby inducing the lac operon. The lac transcriptional regulatory system therefore represents a positive feedback loop, such that increasing concentrations of lactose will tend to maximize lac operon transcriptional activity.

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Lac Operon Genetics Practical.  (2013, March 6).  Retrieved April 13, 2021, from

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"Lac Operon Genetics Practical."  March 6, 2013.  Accessed April 13, 2021.