ara operon
The arabinose operon , or ara operon for short, is an operon that plays an important role in the breakdown of L-arabinose in bacteria such as Escherichia coli . It consists of two promoters (P C and P BAD ), two operators (araO 1 and araO 2 ), an inducer (araI) consisting of I 1 and I 2 , and three structural genes:
- araA (arabinose isomerase; converts arabinose to ribulose )
- araB (ribulokinase; phosphorylates the ribulose to ribulose-5-phosphate )
- araD (ribulose-5-phosphate epimerase; converts ribulose-5-phosphate into xylulose-5-phosphate , which is processed via the pentose phosphate route )
functionality
The ara operon has four important regulatory regions that are regulated by the regulatory protein AraC (encoded by araC ), which can act as both an anti-activator and an activator:
- araO 1 : is an operator; theregulatory protein AraC encodedby araC can bind here and suppresses the synthesis of the mRNA that encodes the regulatory protein (also called C protein). So it is a negative regulation: The C protein is a negative regulator of araC, ie it inhibits the formation of itself (provided there is no arabinose).
- araO 2 : is also an operator; if the regulatorprotein isboundboth to this site and to araI (more precisely: I 1 ), transcription on the promoter P BAD is suppressed. In the absence of arabinose, AraC binds preferentially to araO 2 and I 1 , creating a DNA loop thatinactivatesP C. In addition, AraC cannotbindto I 1 and I 2 , so that P BAD cannot be activated.
- araI : is an inductor area; in the presence of arabinose, AraC supportsthe activation of transcription at the promoter P BAD at this point (more precisely: I 1 and I 2 ). In addition, the promoter P C is now accessible to the RNA polymerase and can be read off.
- CAP : CAP binding site; in the absence of simple sugars such as glucose, CAP initiates the rearrangement of the C protein and prevents the formation of DNA loops and thus activates transcription at the P BAD promoter .
araC suppresses its own expression to avoid excessive accumulation. This mechanism is known as autoregulation. If the concentration of araC falls, araO 1 is released and araC transcription begins again until a sufficient concentration value is reached.
In the presence of arabinose , it is bound by the regulator protein. Due to allosteric changes in the regulator protein, it only binds to araI and no longer to araO 2 . In addition, if there is no glucose present, the CAP protein binds to the CAP binding site and prevents the formation of DNA loops and facilitates the binding of the regulatory protein to araI. If neither C protein nor cAMP-CAP complex is present, or if C protein is present but no cAMP-CAP complex, no transcription is initiated.