Contrast the role of the repressor in an inducible system and in a repressible system.

Inducible systems are gene regulatory systems that are usually turned off, but can be activated by the presence of an inducer molecule. On the other hand, repressible systems are gene regulatory systems that are usually turned on, but can be deactivated by the presence of a co-repressor molecule. Both systems involve a repressor protein that modulates the activation or repression of gene expression.

In an inducible system, the repressor protein is produced in an active form, which can bind to a specific DNA sequence (the operator) near the gene that needs to be regulated. When the repressor is bound to the operator, it blocks the RNA polymerase from transcribing the gene, effectively turning it off. When an inducer molecule is present, it binds to the repressor, causing a conformational change that makes the repressor inactive and unable to bind to the operator. This allows the RNA polymerase to transcribe the gene, turning it on. Examples of inducible systems include the lac operon in bacteria, where the lac repressor protein binds to the operator to repress the expression of genes involved in lactose metabolism, and the presence of lactose as an inducer molecule leads to the inactivation of the repressor and the activation of gene expression.

In a repressible system, the repressor protein is produced in an inactive form, which cannot bind to the operator sequence. The genes are transcribed, and their expression is turned on by default. When a co-repressor molecule is present, it binds to the repressor, causing a conformational change that makes the repressor active and able to bind to the operator. This blocks the RNA polymerase from transcribing the gene and turns the gene expression off. Examples of repressible systems include the trp operon in bacteria, where the trp repressor protein is normally inactive, allowing the expression of genes involved in tryptophan synthesis. When tryptophan levels are high, tryptophan acts as a co-repressor and binds to the trp repressor protein, activating it and repressing gene expression.

In summary, the role of the repressor in an inducible system is to block gene expression by default, until an inducer molecule is present that inactivates the repressor and allows gene expression. In a repressible system, the repressor is inactive by default, allowing gene expression until a co-repressor molecule is present and activates the repressor, which then blocks gene expression. Essentially, the repressor acts as a switch that can be turned on or off depending on the presence of specific molecules (inducer and co-repressor) in the two different regulatory systems.