Types of Gene Regulation

Gene regulation can occur at various steps. The amount of product depends on:

• rate of mRNA synthesis (transcription),
• mRNA degradation,
• protein synthesis (translation) etc.

Prokaryotes commonly control transcription

• Constitutive genes are always expressed
• • Tend to be vital for basic cell functions (often called housekeeping genes)
• Regulated genes can be inducible or repressible
• • Inducible genes are normally off, but can be turned on when substrate is present
  • • Common for catabolic genes (i.e. for the utilization of particular resources)'
    • The lac operon is inducible.
  • Repressible genes are normally on, but can be turned off when the end product is abundant
  • • Common for biosynthesis genes

More Terminology

Repressors and Activators are proteins that bind to DNA and control transcription. Those genes are said to be repressible or inducible.

Inhibitors and Inducers are small "effector" molecules that bind to repressors or activators.

Though there is lots of terminology, the concepts are pretty simple: various factors interact to turn things on or off. For example, repressors simply act ans an obstacle to block transcription. If another molecule (the inducer) interferes with the repressor, they don't bind DNA and transcription proceeds normally.

Operons

• In Prokaryotes, functionally related genes are regulated as a unit, called an operon.
• Operons consist of:
• • Several structural genes
  • ONE promoter and one terminator
  • A control site (operator)
  • A separate regulator gene (codes for protein that binds to operator)

Overview of the lac Operon

• Gene is normally off
• There is no transcription because a repressor binds to the control site
• When lactose is present, it inactivates the repressor, allowing transcription to begin. That produces both lacZ (to break down lactose) and lacY (to let it into the cell)
• When lactose is used up, the repressor is again free to bind to DNA, and halt transcription.
• Glucose must be absent. If glucose is present, transcription doesn't start.

Cells respond quickly to available sugars:

B-galactosidase activity increases almost immediately upon transfer to lactose as a sole carbon source, and stops rapidly after the addition of glucose.

See and study Fig. 16.5 and 16.7 to see how the normal lac operon responds to different types of sugars.

Remember that it is probably better to learn the concepts of "repressor" and "inducible gene" and "operon" and then use logic to apply that to lac, rather than just memorizing the genes.

Polar mutations

In polycistronic mRNA, anything that prematurely stops translation will also affect all other genes downstream in the operon

Using genetics to figure out the lac operon

• Jacob and Monod identified several mutations in genes of the lac operon
• lacZ^-, lacO^c, etc.
• used F' strains that had various combinations of mutant and wild type genes
• Those F' strains were "partial diploids" for the lac operon.

First look at single mutations:

What effects do you predict for mutations in B-galactosidase, lacZ? As an approach, ask these kinds of questions:

• What does lacZ do?
• What will be the consequence if it is non-functional?
• Will the mutation be dominant or recessive in partial diploids?

In this case, a mutant lacZ will not be able to metabolize lactose, whether or not the gene is induced. But it will be recessive: if there is another functiona; copy on the plasmid, B-gal. should be produced normally.

What about the repressor locus, lacI?

• What effects do you predict for mutations in the regulator gene lacI^-?
• What does lacI do?
• What will be the consequence if it is non-functional?
• Will the mutation be dominant or recessive?

See Fig. 16.9 which shows the effects of a lacI^- mutation

Mutations in the regulatory gene lacI affect the genes on both DNA molecules, so it is trans-dominant. That is typical of binding proteins and other diffusible substances.

Test of the model

Jacob and Monod predicted there would be a third type of mutation: mutations in the operator, lacO

• What effects should those have?
• What does the operator do?
• What will be the consequence if it is non-functional?
• Will the mutation be dominant or recessive?

Fig. 16.8 shows effects of operator mutations.

Mutations in the operator only affect the genes linked to it on the same DNA molecule, so they are said to be cis-dominant. This is typical of transcription control elements like promoters, etc.

Another lacI mutation: super-repressors (lacI^s)

In those mutants, the repressor is always active, whether or not the inducer (allolactose) is present.

Remember, repressors have two sites: one to bind DNA and the other to bind allolactose. Which site do you think is mutated in lacI^s

What about glucose?

If glucose is present, lac genes remain silent

A positive activator is needed

CAP-cAMP, helps RNA polymerase bind to the promoter

If glucose is low, cAMP builds up in cells and activates CAP

The CAP-cAMP system is a separate regulator of lac, which operates by a completely different mechanism. Rather than blocking transcription, it is an activator that is required for efficient binding of the promotoer to the DNA.