Written by Lori Rowan and Joe Keller
The lactose and arabinose operons share many similarities. First, both have the same basic structure. Each contains a promoter, regulatory protein, operator region, and structural genes. The regulator protein in the lactose operon is lacI and the regulator proteins in the arabinose operon are araI, araO2, araO, and cAMP. The operator region in the lactose operon is O1, O2, and O3 and in the arabinose operon the operator region is araO1 and araO2. They also each contain three structural genes. In the lactose operon they are lacZ, lacY, and lacA. In the arabinose operon they are araA, araB, and araD.
Second, both operons are inducible. According to Gene Mayer of the University of South Carolina School of Medicine, inducible operons “are those in which the presence of a substance (an inducer) in the environment turns on the expression of one or more genes (structural genes) involved in the metabolism of that substance”. Thus, the structural genes in lactose operon and arabinose operon are normally inactive.
Third, when glucose is present at high levels it prevents both the synthesis of arabinose and lactose. This occurs because glucose lowers the levels of cAMP which prevents the CAP-cAMP complex to form. In both the lactose and arabinose operons, the CAP-cAMP complex is needed in order to undergo positive regulation and allow the synthesis of lactose and arabinose.
Fourth, the functions of both the lactose operon and arabinose operon are parallel. They both undergo negative regulation. During negative regulation in the lactose operon, the repressor binds to the operator region. This prevents RNA polymerase from being able to bind, which inhibits transcription of lactose. During negative regulation in the arabinose operon, there is little CAP-cAMP present which causes araC to simulate the inducer and operator to form a loop. This prevents the RNA polymerase from binding to the DNA and blocks the transcription of arabinose.
Fifth, each also undergoes positive regulation. In the lactose operon, when lactose is present the CAP-cAMP complex forms and binds to the DNA. This binding of the CAP-cAMP complex to the DNA, causes a change in the conformation of the DNA which prevents the repressor from binding. Since the repressor is prevented from binding, this allows the RNA polymerase to bind. The RNA polymerase then allows transcription to begin in order to make lactose. In the arabinose operon, when arabinose is present it increases the amount of CAP-cAMP complex formed. When the CAP-cAMP complex is formed, it binds to araI which allows RNA polymerase to bind to the DNA. The binding of RNA polymerase initiates the transcription of arabinose.
Sixth, the products formed in both the lactose operon and arabinose operon are very similar. During negative regulation, no sugar is made in both operons. During positive regulation, both synthesize the correct sugar. In the lactose operon, lactose is synthesized and in the arabinose operon arabinose sugar is synthesized.
Finally, along with producing similar products their conditions for expression are also very alike. Both require cAMP and the correct sugar; either lactose or arabinose, to be present. Additionally, there must be no glucose present and the sugar; either lactose or arabinose, must be the only carbon source available in order to begin transcription.