LAC-OPERON

LAC – OPERON  

                Lac         -   Lactose

                        Operon -   Functional unit of DNA containing cluster of genes under a single promoter.

·       The regulatory mechanism of the lac system, which was elucidated by the elegant genetic  analysis of Jacob and Monad in 1961, for which they got nobel  prize in physiology in 1965.

·       The lactose-operon is also called Inducible operon.

·       The lactose-utilization system consists of two kinds of  components

1.    Structural genes ( needed for transport and metabolism of lactose)

2.    Regulatory elements the lac I (Repressor) gene, the lac O (operator), and the lac P (promoter). Together these components comprise the lac operon.

·         The products of the lac Z and lac Y genes are encoded in a single polycistronic mRNA molecule. ( This mRNA molecule contains a third gene, denoted as lacA ) .

·         Lac Z: The gene  lac Z encodes the enyme beta-Galactosidase, it breaks the  lactose into Glucose and Galactose.

·          Lac Y: Lac Y encodes the enzyme Lactose permease , it is used for transport of more lactose inside the cell.

·          Lac A: Lac A encodes the enzyme Thiogalactoside transacetylase, it removes the toxic thiogalactosides  transported by lac Y.

·         PROMOTER: The promoter for the lac Z, lac Y, lac A mRNA molecule is immediately adjacent to the lac O region,where RNA Polymerase binds and performs transcription.

·         The lac I gene product, the lac repressor, binds to the

operator site.

·         When the repressor is bound to the operator, initiation of transcription of lac mRNA polymerase is prevented.

·         The inducer stimulates lac mRNA synthesis by binding to and inactivating the lac I repressor, a process called either induction or depression. Thus, in the presence of an inducer, the operator is unoccupied, and the promoter is available for initiation of mRNA synthesis.

·         CAP Site: It is a positive regulatory site, were catabolite activator protein (cAMP Receptor Protein- CRP) is bound. The

cAMP binds to the CRP and it gets activated, this complex binds to the CAP site and interacts with the C terminal domain of α subunit of  RNA polymerase enzyme  and increase transcription of lac genes.

·       Allolactose: It binds to an allosteric site on the repressor protein causing a conformational change. As a result of this, the repressor can no  longer bind to the operator region and it falls off, aids  in transcription of lac genes.

  REFERENCE :

              Molecular biology of the gene, 7th edition by James Watson and Francis Crick.

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           Dr. A. P. J. Abdul Kalam.