Controlling Gene Expression

Web Lecture

• Operons
• RNA transcription

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Lecture: How cells control gene expression

Note that the examples of operons in your book are drawn from studies of the E. coli bacteria, a prokaryotic cell. These examples occur in a number of texts because they are fairly well understood (by which we really mean that the model we have accounts for most of our observations). We do not understand the operation of gene expression in more complex organisms nearly as thoroughly.

Using operons

Think of some project for which you need supplies, such as baking a cake. You can go to the store and get eggs and sugar, come home and cream them together, then go back to the store to get flour and spices and sift them together, then go back again for the milk and oil—OR—you can go to the store once, get all the ingredients and come home, ready to make a cake without further trips to the store (and the gas station!).

Having DNA organized into operons makes it possible for the cell to reduce the number of "trips to the nucleus" to get the mRNA it needs to make various enzymes. Operons are strings of related genes which share a single operator site and stop nucleotide. This means that when the RNA polymerase attaches to the promotor site, it creates mRNA for all the genes on the operon as a single mRNA unit. Cells use operons to efficiently organize enzyme production. If a particular situation (such as the breakdown of lactose into glucose) calls for a series of reactions, each with its own enzyme catalyst, then the cell needs to create all of the enzymes required, not one or two.

Controlling mRNA production at the start of the process

In normal gene expression, an RNA polymerase molecule latches onto the promoter site and "walks" the DNA to the gene, where it replicates the genetic information as mRNA that can then leave the nucleus and create an appropriate enzyme molecule. The mRNA needed for a particular function can be produced only when the RNA polymerase can attach to the gene's promotor site (whether it is the operator for a single enzyme or for a set of enzymes in an operon doesn't matter).

A cell can control the production of a protein by regulating the production of the mRNA that codes for that protein, and one way to do that is to keep the RNA polymerase from attaching to its promoter site. Promotor sites are separated from the genes they "promote" by a series of nucleotides which form an operator site. A repressor molecule may be able to attach to the promoter site and block the operation of the RNA polymerase. If the operator site is occupied by a repressor molecule, the shape of the promotor site is changed and the RNA polymerase cannot attach.

Some genes (like the one pictured) are constitutive: their promotor sites are always available to the RNA polymerase, so that mRNA is continually made for them. Many repressor molecules are constitutive, so the repressor is made constantly, and the genes they control are always suppressed -- unless something happens to change the repressor molecule. A repressor can be turned off (and made unable to bind to its operator site) by another molecule. In E. coli bacteria, the repressor gene for the lactose operon is made inactive by a form of lactose. So if lactose is present in the cell, the repressor is turned off, and the genes to break down lactose are manufactured until all the lactose available is broken down, including the lactose pulled off the repressor. Then the repressors are free to bind to the operon operator, and lactose gene production stops again. Genes which are expressed only when their repressor is turned off are called inducible genes.

Other constitutive genes can be repressed only if the repressor molecule for their own operator site is activated by some condition in the cell, such as an excess of the protein which they are made to suppress. In this situation, the repressor as it is made is not able by itself to attach to the operator site. Instead, the RNA polymerase attaches to the promotor site, and the genes are transcribed to mRNA which creates the specified proteins. These proteins then bind to the repressor gene, which is now able to bind to the operator site and prevent the further transcription of mRNA for that protein. Manufacture of the protein stops, levels of the protein in the cell drop, the proteins attached to the repressor cells are liberated, and the repressor cells detach from the operator, so that the whole cycle can start again. Genes controlled by this mechanism are repressible.

Type of Expression	Regulation	Description	Type of Gene	Example
Constitutive	Unregulated	Genes always ON	Housekeeping genes	Ribose production proteins and components
Inducible	Regulated	Gene turned on as needed	Structural genes	Glucose transport proteins
Repressible	Regulated	Gene turned off as needed	Structural genes	TRP (bacteria operon)
Silenced	Regulated	Gene turned off	Trait unneeded by cell	Neurotransmitter proteins off in muscle cells

Activator proteins can speed up enzyme production by lowering the amount of energy required to attach the RNA polymerase to its promotor site. Activator proteins often attach to the DNA near the promotor site, changing the shape of the promotor so that it more readily accepts the RNA polymerase.

A final type of gene regulation results from cell differentiation. In this case, a gene is permanently turned off and won't be expressed by the cell, since it is inappropriate carry out cell functions. For example, eye color pigmentation genes are activated only in iris cells, but not in skin or heart muscle cells.

All four mechanisms discussed so far control the rate of translation, the creation of mRNA from the DNA template. Often the sequence of reactions for a particular purpose is controlled by many enzymes, each of which is itself controlled by some combination of induction, repression or activation mechanisms. This flexibility allows the cell to fine tune its performance, and to move from one process to another easily, depending on the amount of materials and energy it has at its disposal.

X Chromosome inactivation

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