The success of the Operon Model of gene expression in explaining the observed phenotypes of sugar metabolism convinced Monod that all regulation was negative regulation.
However, at the same time as he was developing the model to explain lactose metabolism, Ellis Englesberg was studying the metabolism of arabinose in E. coli. Englrsberg did all the proper genetic analysis and came to the conclusion that expression of the arabinose operon was regulated in a positive manner. It took quite some time before Englesberg's hypothesis was validated.
The Arabinose Operon
The ara operon codes for three enzymes that are required to catalyze the metabolism of arabinose.
- Arabinose isomerase - encoded by araA - coverts arabinose to ribulose
- Ribulokinase - encoded by araB -- phosphorylates ribulose
- Ribulose-5-phosphate epimerase - encoded by araD -- converts ribulose-5-phosphate to xylulose-5-phosphate which can then be metabolized via the pentose phosphate pathway.
The three structural genes are arranged in an operon that is regulated by the araC gene product. There are four important regulatory sites as shown in the following diagram:
- araO1 is an operator site. AraC binds to this site and represses its own transcription from the PC promoter. In the presence of arabinose, however, AraC bound at this site helps to activate expression of the PBAD promoter.
- araO2 is also an operator site. AraC bound at this site can simultaneously bind to the araI site to repress transcription from the PBAD promoter
- araI is also the inducer site. AraC bound at this site can simultaneously bind to the araO2 site to repress transcription from the PBAD promoter. In the presence of arabinose, however, AraC bound at this site helps to activate expression of the PBAD promoter.
- CRP binds to the CRP binding site. It does not directly assist RNA polymerase to bind to the promoter in this case. Instead, in the presence of arabinose, it promotes the rearrangement of AraC when arabinose is present from a state in which it represses transcription of the PBAD promoter to one in which it activates transcription of the PBAD promoter.
Regulation of the arabinose operon is, clearly, much more complex than the lactose operon.
When arabinose is absent, there is no need to express the structural genes. AraC does this by binding simultaneously to araI and araO2. As a result the intervening DNA is looped. These two events block access to the PBAD promoter which is, in any case, a very weak promoter (unlike the lac promoter):
AraC also prevents its own expression. Thus, it is an autoregulator of its own expression. This makes sense; there is no need to over-express AraC. If the concentration falls too low then transcription of araC resumes until the amount of AraC is sufficient to prevent more transcription again.
When arabinose is present, it binds to AraC and allosterically induces it to bind to araI instead araO2. If glucose is also absent, then the presence of CRP bound to its site between araO1 and araI helps to break the DNA loop and also helps AraC to bind to araI:
The ara operon demonstrates both negative and positive control. It shows a different function for CRP. It also shows how a protein can act as a switch with its activity being radically altered upon the binding of a small molecule.