Question

In: Biology

Mutations affecting methionine metabolism have been isolated in E. coli. An analysis of 4 strains in...

Mutations affecting methionine metabolism have been isolated in E. coli. An analysis of 4 strains in the presence and absence of methionine is presented below. A + indicates that methionine-producing enzymes are present; a - indicates that methionine-producing enzymes are not present.

Strain.

1. metJ+ metA+

2. metJ- metA+

3. metJ+ metA-

4. metJ- metA-

Methionine Present

-

+

-

Methionine

Absent

+

-

A. Examine strain 1, the wild type. Is the methionine operon most likely an example of an inducible or a repressible gene regulatory system? Explain.

B. Examine strains 2, 3, and 4. Which gene metJ or metA is the structural gene? the regulatory gene? Explain briefly.

C. Is this operon system an example of a positive or a negative regulatory system? Explain.

D. In the presence of methionine, the cell has a high concentration of S-adenosyl-methionine (SAM). In the absence of methionine, SAM is in a low concentration. SAM is an effector molecule that binds the regulatory protein. Is SAM an inducer or a co-repressor? Explain.

Expert Solution

A. Examine strain 1, the wild type. Is the methionine operon most likely an example of an inducible or a repressible gene regulatory system? Explain.

The methionine operon is most likely a repressible gene regulatory system. It is so since in the presence of methionine the production of methionine enzymes are repressed.

Here metJ acts as a regulatory gene and metA acts as the structural gene.

A structural gene is any gene that codes for a protein product or mRNA other than the regulatory protein

A regulatory gene is one which is involved in the control of the expression of one or more genes

When methionine is present, there is no production of methionine even in the presence of both metJ and metA genes. This implies that either one of them is a repressor and represses the transcription of the other.

When methionine is present, there is production of methionine producing enzymes in Strain 2. Here we see that metA is present and metJ gene is absent .Thus we can conclude that metA gene is the structural gene and codes for the methionine producing enzyme.

When methionine is absent, we see there is production of enzymes in strain 1 . From this we can conclude that , since there is no co-repessor (Methionine) the regulatory gene product of metJ which is a repressor protein is unable to bind metA and thus there production of enzymes.

Thus we can conclude that this is a repressible gene regulatory system.

A repressible gene regulatory system or operon is defined as one that is regulated by a co-repressor which inhibits the expression of the gene system

B. Examine strains 2, 3, and 4. Which gene metJ or metA is the structural gene? the regulatory gene? Explain briefly.

metJ is the regulatory gen and met A is the structural gene.

When methionine is present, there is production of methionine producing enzymes in Strain 2. Here we see that metA is present and metJ gene is absent .Thus we can conclude that metA gene is the structural gene and codes for the methionine producing enzyme

In strain 2, there is production of enzymes when methionine is absent. Here metJ is missing . So we can conclude metA is the structural gene.

No conclusion can be derived from strain 3 and 4 since metA is absent in both. Thus there will be no production of enzymes in the absence or presence of methionine in the medium

C. Is this operon system an example of a positive or a negative regulatory system? Explain.

It is a negative regulatory system.

Reason :

In the presence of methionine, the expression of methionine producing enzymes is repressed or slowed down. Here the product is inhibiting the formation of product. This is a classic example of a negative regulatory system.

A negative regulatory system refers to one where a repressor protein binds to the operator and inhibits the expression of the structural gene.

D. In the presence of methionine, the cell has a high concentration of S-adenosyl-methionine (SAM). In the absence of methionine, SAM is in a low concentration. SAM is an effector molecule that binds the regulatory protein. Is SAM an inducer or a co-repressor? Explain.

We have observed that in the presence of methionine , the expression of structural gene metA is repressed and the production of methionine producing enzymes is decreased or absent. Due to this, the increase in effector molecule also results in repression of the system. Thus SAM acts as a co-repressor which binds the regulatory protein to inhibit the transcription of the structural gene metA.

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