a bottle of milk

The Lactose & Arabinose Operons

The Arabinose Operon

a diagram of the arabinose operon
      The three structural genes (araB, araA, araD) encode for enzymes needed for the metabolism of the sugar arabinose in bacterial cells. AraB, araA, and araD encode for the enzymes kinase, isomerase, and epimerase. Isomerase converts arabinose to ribulose. Kinase converts ribulose to ribulose-5-phosphate. Then, epimerase converts ribulose-5-phosphate to xyulose-5-phosphate, which enters the glycolytic pathway for carbon metabolism.

      The arabinose operon also contains the arabinose C gene, which synthesizes itself. The araC gene regulates the expression of the structural genes and the araC gene product. Thus, the araC gene is auto regulated. When the level of araC gene product is high in the cell, the araC gene binds to its operator, preventing further synthesis of the araC gene product.
c-gene product bound to araC
The presence of both arabinose and the araC gene product activates the expression of the BAD genes. Arabinose binds to the araC gene product, causing a change in the shape 5 . This new shape helps RNA polymerase to bind to the BAD promoter, which sends a signal to begin the transcription of the structural genes 5 . The araC gene acts as a positive control for the BAD genes and a negative control for its own synthesis.
arabinose bound to c-gene product
      Mutations at the araA or araB genes block the pathway of arabinose metabolism. Bacterial cells can survive with the presence of arabinose, but can not utilize it. If there is a mutation at the araD gene, the cells will not survive due to the build up of L-ribulose-5P, which is toxic to the cell. Mutation of the araC gene prevents the synthesis of the araC gene product. As a result, the structural genes are not expressed.

References
  1. oregonstate.edu - E. coli Gene Regulation
  2. The Board of Trustees of the Univeristy of South Carolina - Genetic Regulatory Mechanisms
  3. Bookrags.com - "Operon"
  4. Caruso, S.M, et al. Genetics Lab Manual, Bio 302L, Fall 2005
  5. Freeman, W. H. and Company. "Dual Positive And Negative Control: the Arabinose Operon"

This page was written to fulfil the requirements of Genetics Lab at UMBC