Objectives
By the end of this lesson you should be able to:
- Describe the operation of the trp operon
- Describe the operation of the lac operon
- Differentiate between positive and negative gene regulation
- Give an example of positive gene regulation
What’s An Operon?
Metabolic controls generally take place at two levels. The organism can either (1) control enzyme activity, or (2) control enzyme production. Two examples of regulation by the latter is observed in the trp and lac operon. Both mechanisms are utilized by Escherichia coli bacteria to conserve resources. An operon is a cluster of genes that are transcribed together by a single mRNA. The promotor site has an operator (an on/off switch) that determines whether or not RNA is able to transcribe the proteins. All proteins must be transcribed together since they all play a role in the metabolism of the specific nutrient to be metabolized.
How the Trp Operon Works
An upstream regulatory gene called TrpR is responsible for the production of a trp repressor protein. The protein is made in its inactive form, but when there is enough tryptophan available, tryptophan binds to the repressor to cause a conformational change. This change enables the repressor to bind to the DNA at the operator site, blocking the activity of RNA polymerase. Thus, transcription of the proteins that are needed to synthesize tryptophan is blocked. After all, why waste resources to make tryptophan if enough tryptophan is available in the cell for “free”?
The trp operon is a repressor operon since its transcription is usually on but is repressed when trp is available. In contrast, an inducible operon is one that is usually off but must be induced with a certain molecule to turn it on. This is the case of the lac operon.
How the Lac Operon Works
The purpose of the Lac operon is to produce enzymes necessary for the breakdown and utilization of lactose. The enzymes involve are beta-galactosidase, permease, and galactosidase transacetylase. Permease enables the entry of lactose into the cell. Beta-galactosidase breaks down lactose to glucose and galactose. The role of transacetylase is ill-defined but is also involved in lactose metabolism, primarily in the transfer of acetyl group from acetyl-CoA to galactose.
The transcription unit is under the direction of the LacI regulatory gene located upstream of the operon. LacI provides the transcript for the production of an active repressor protein that attaches to the operator and blocks RNA polymerase. However, when lactose is available, an inducer called allolactose (an isomer of lactose) binds to the active repressor making it inactive. This causes it to dissociate from the operator, allowing RNA polymerase to transcribe the lactose-metabolizing proteins.
Both the trp operon and the lac operon are examples of negative control of genes since the active repressors have a negative effect on transcription.
Gene regulation may also be accomplished by positive control. This occurs when a regulatory protein interacts directly with the gene to switch on transcription. We can look at an example involving the lac operon.
So, we have seen so far that in the lac operon has an on/off switch. In a way, it also has a volume switch. When glucose is scarce, a molecule called cyclic AMP (cAMP) accumulates in the cell. This molecule attaches to a regulatory protein called catabolic activator protein (CAP) which acts as a transcription activator when it binds to the DNA. Once it binds, RNA polymerase has a higher affinity to the DNA, causing transcription rate to be ramped up (a ‘volume’ increase).
When glucose is abundant, there is less cAMP around. Therefore, there are fewer active CAP available. This causes transcription rate to drop reducing the need to metabolize lactose.
Reference: Reece, J. B., & Campbell, N. A. (2011). Campbell biology. Boston: Benjamin Cummings / Pearson.
