Negative and Positive Control: Inducible and Repressible Operons

Negative and Positive Control: Inducible and Repressible Operons

The regulation of gene expression (induction and repression) can be accomplished by both positive control mechanisms and negative control mechanisms. Both mechanisms involve the participation of regulator genes.

 Regulation of an operon (i.e. expression of the structural genes) is brought about by two elements:

1) cis-acting DNA sequences (promoter and operator/activator binding sequences) which form a part of the operon

2) trans-acting protein molecules (activators or repressors) that bind to the cis-acting sites.

In positive control mechanisms, the product of the regulator gene (repressor) is required to turn on the expression of one or more structural genes (genes specifying the amino acid sequences of enzymes or structural proteins)

In negative control mechanisms, the product of the regulator gene is necessary to shut off the expression of structural genes.

Theoretically, negative control could be inducible or repressible.

Negative inducible operons

 Negative regulation: The control at the operator site is negative (The binding of the regulator protein to the operator inhibits transcription)

The regulatory protein act as a repressor at the operator site

Inducible operon: Operons that are usually off and need to be turned on, so the transcription is inducible.

Inducer: small molecule that turns on the transcription

 How the operon behave when no precursor is present (Figure 1a)

  • In a negative inducible operon, transcription and translation of the regulator gene produce an active repressor that readily binds to the operator.
  • Because the operator site overlaps with the promoter site, the binding of this protein to the operator physically blocks the binding of RNA polymerase to the promoter and prevents transcription of structural genes.

(For transcription to take place, something must happen to prevent the binding of the repressor at the operator site. Transcription is turned on when a small molecule called an inducer binds to the repressor).

How the operon behave when the precursor is present (Figure 1b)

Regulatory proteins frequently have two binding sites:

1) For binding to DNA at the operator site

2) For binding to small effector molecule (inducer)

Precursor V acting as the inducer here. Binding of the inducer to the repressor alters the shape of the repressor. As a result the repressor can no longer bind to the operator.

(Proteins of this type which change shape on binding to another molecule are called allosteric proteins).

This is an adaptive mechanism because when no precursor V is available, it would be wasteful for the cell to synthesize the enzymes needed to metabolize the precursor. As soon as precursor V becomes available, some of it binds to the repressor, rendering the repressor inactive so that it unable to bind to the operator site. Now RNA polymerase can bind to the promoter and transcribe the structural genes. The resulting mRNA is then translated into enzymes D, E, and F, which convert substrate V into product W.

(So an operon with negative inducible control regulates the synthesis of the enzymes economically: the enzymes are synthesized only when their substrate (V) is available)

Example: The lac operon of E. coli

  • The lac operon in Escherichia coli was the first-discovered operon model and is a characteristic example of a negative inducible operon (derepressible model).
  • Allolactose acts the inducer molecule that binds to the repressor protein called as lac repressor produced by the lacl gene (the regulatory gene of lac operon) and switches on the operon to transcribe the gene.
Figure 1: Negative inducible operon

Negative repressible operon

 Negative regulation: The control at the operator site is negative (The binding of the regulator protein to the operator inhibits transcription)

The regulatory protein act as a repressor at the operator site

Repressible operon: Operons in which transcription normally takes place and must be turned off or repressed.

The regulator protein in this type of operon also is a repressor but is synthesized in an inactive form that cannot by itself bind to the operator. As a result RNA polymerase readily binds to the promoter and transcription of the structural genes takes place.

To turn transcription off, something must happen to make the repressor active. A small molecule called a co repressor binds to the repressor and makes it capable of binding to the operator.

How the operon behaves when product U is present (Figure 2b)

  • The product (U) of the metabolic reaction is the corepressor.
  • As long as the level of product U is high, it is available to bind to the inactive repressor. Their binding will activate the repressor.
  • Active repressor will bind to the operator and prevents transcription of structural genes (operon repressed).
  • As a result of this enzymes G, H, and I are not synthesized, and no more U is produced from precursor T.

How the operon behaves when product U is absent (Figure 2a)

  • When all of product U is used up, the repressor is no longer activated by U and cannot bind to the operator.
  • The inactivation of the repressor allows the transcription of the structural genes and the synthesis of enzymes G, H, and I, resulting in the conversion of precursor T into product U.
  • Repressible operons are also economical: Because the enzymes are synthesized only as needed
  • (Note that both the inducible and the repressible systems that we have considered are forms of negative control, in which the regulatory protein is a repressor).

Example: Trp operon in E.coli

  • E.coli can synthesize tryptophan using enzymes that are encoded by five structural genes located next to each other in the trp operon.
  • When environmental tryptophan is low, the operon is turned on. This means that transcription is initiated, the genes are expressed, and tryptophan is synthesized.
  • However, if tryptophan is present in the environment, it acts as a co repressor and the trp operon is turned off. Transcription does not occur and tryptophan is not synthesized.

Formation of amino acid tryptophan needs action of five enzymes in succession. The formation of tryptophan is on because regulator gene R forms an inactive repressor called aporepressor which does not attach itself to operator site. As operator site is free of repressor, the operon system remains on leading to the synthesis of all five enzymes needed for tryptophan formation.

When tryptophan accumulates, a few molecules (of tryptophan) act as co-repressor and bind to inactive repressor activating it. On activation this attaches to the operator, switching off the operon in turn.

Figure 2: Negative repressible operon

Positive regulation

  • In positive control mechanisms, the product of a regulator gene (an activator) is required to turn on the expression of the structural gene(s).
  • Regulatory protein binds to DNA (usually at a site other than the operator) and stimulates transcription.

Theoretically, positive control could be inducible or repressible.

Positive inducible operon

  • In a positive inducible operon, transcription would normally be turned off because the regulator protein would be produced in an inactive form.
  • Transcription would take place when an inducer became attached to the regulatory protein, rendering the regulator active.
  • Logically, the inducer should be the precursor of the reaction controlled by the operon so that the necessary enzymes would be synthesized only when the substrate for their reaction was present.

Figure 3: Positive control of inducible gene expression

Positive repressible operon

  • In a positive repressible operon, transcription would normally take place and would have to be repressed. In this case, the regulator protein would be produced in a form that readily binds to DNA and stimulates transcription.
  • Transcription would be inhibited when a substance became attached to the activator and rendered it unable to bind to the DNA so that transcription was no longer stimulated.

Here, the product (P) of the reaction controlled by the operon would logically be the repressing substance, because it would be economical for the cell to prevent the transcription of genes that allow the synthesis of P when plenty of P is already available

Figure 4: Positive control of repressible gene expression

How to remember the basics?

Theoretically, operons might exhibit positive or negative control and be either inducible or repressible. Try sketching out all possible types— negative inducible, negative repressible, positive inducible, and positive repressible.

To do so, learn the meanings of positive and negative control and inducible and repressible, then use logic to work out the details of whether the regulatory protein is a repressor or an activator and whether it is produced in an active or inactive form.

In order to understand the details of these four mechanisms of regulation, focus on the key differences between them.

(1) The regulator gene product (the activator) participates in turning on gene expression in a positive control mechanism, whereas the regulator gene product (the repressor) is involved in turning off gene expression in a negative control mechanism.

(2) With both positive and negative control mechanisms, whether gene expression is inducible or repressible depends on whether the free regulator protein or the regulator protein/effector molecule complex binds to the regulator protein-binding site (RPBS).

 

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