An Example of Control in Prokaryotic Cells (Bacteria)

The Lac Operon

We are going to go into some detail to describe how a bacteria (E. coli) commonly found in the human digestive tract regulates the metabolism of the sugar lactose that is found in milk. It might seem strange that we are going to study one example in such detail. Scientists will often use what are called "model systems" to study complex situations. They reason that if they can understand one system in detail, they can then extrapolate their findings to other systems.

The regulation of the genes for metabolism of lactose is one of the best understood systems of regulation of gene expression (i.e. it's a "model system"). Control of gene expresssion can be exerted at several different points such as at transcription of DNA into mRNA and at translation.

In prokaryotes such as bacteria, no nucleus separates DNA from ribosomes in the cytoplasm. When nutrient supply is high, transcription proceeds rapidly. Translation occurs even before mRNA transcripts are finished. Seventy-five different operons controlling 250 structural genes have been identified for E. coli. Can you spot the location of the lac operon on this bacterial chromosone gene map?

Negative Control of the Lactose Operon

E. coli bacteria can metabolize lactose because of a series of genes that code for lactose-digesting enzymes. Since not everyone has milk in their digestive system at all times, E. coli would be wasting its resources if it continually produced the proteins needed for lactose metabolism.

The lactose operon is composed of three genes coding for different proteins, plus a promoter gene and an operator gene. A regulator gene nearby codes for a repressor protein that binds to the operator when lactose concentrations are low and effectively blocks RNA polymerase's access to the promoter. Transcription is blocked

This animation (Audio - Important) describes the lac operon.

When milk is consumed and lactose levels are high , the lactose binds to the repressor changing its shape and effectively removing its blockage of the promoter. Then RNA polymerase can initiate transcription of the genes.

Here is a lac operon animation from the University of Virginia.

Take a look at the lac operon tutorial.

These two animations (Audio - Important) review negative control of the lac operon:
negative control of the lac operon 1,
negative control of the lac operon 2.

Positive Control of the Lactose Operon

Let's add one more step of complexity. E. coli would really rather use glucose than lactose. If lots of glucose is present, it won't utilize lactose even if it's present in high concentrations.

So, in the case of low glucose , an activator protein called CAP becomes active and forms a complex with cAMP and turns on the lactose metabolism genes.. CAP will adhere to promoter only when in complex with cAMP.

When glucose levels are high and there is little cAMP, CAP cannot be activated. The promoter is not good at binding RNA polymerase. The lactose-metabolizing genes are not transcribed very much.

When glucose levels are low, cAMP accumulates. The CAP-cAMP complex forms and binds to the promoter. RNA polymerase can now bind and the lactose-metabolizing genes are transcribed rapidly.

This animation (Audio - Important) reviews positive control of the lac operon.

REVIEW: The lactose operon includes

REVIEW: The obvious advantage of the lactose operon system is that

REVIEW: The positive control of the lactose operon in bacteria is

REVIEW: A base sequence signalilng the start of a gene is a(n) _____.

REVIEW: In prokaryotic cells but not eukaryotic cells, a(n) _____ precedes the genes of an operon .

REVIEW: An operator most typically governs _____ .

REVIEW: Eukaryotic genes guide _____ .
a. fast short-term activities
b. overall growth
c. development
d. all of the above

REVIEW: Which of the following is the region that is the binding site for RNA polymerase?

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