Introductory Microbiology

The operon model of gene expression

 
• In bacteria, a group of co-ordinately regulated structural genes with related metabolic functions and the promoter and operator sites that control transcription are called an operon. It is also defined as a cluster of genes whose expression controlled by a single operator. An operon is essentially consisting of following components.

Suppression and Expression

1) Promoter: A site on the DNA where the RNA polymerase binds and begin the protein synthesize.
2) Operator: A site on the DNA where the repressor protein binds and blocks the mRNA synthesis .
3) Repressor protein: A protein bind in the operator region and block the mRNA synthesis.
4) Structural genes: They are the actual coding region of the proteins or enzymes. This protein of the gene is also referred as open reading frame (orf), because this protein alone is read by RNA polymerase to transcript the mRNA.
5) Terminator: This portion of DNA of the operan will terminate the mRNA synthesis by forming loop like arrangement in the same strand.
6) Ribosome binding site: Some 6 base pairs next to promoter, where ribosome bind with mRNA during translation process.

These are the components of any operon. As a model, the diagrammatic representation of lactose utilizing genes, referred as lac operon was given in the below diagram.

P denotes the promoter region, in which the RNA polymerase binds, O denotes the operator region, in which suppressor protein binds and blocks; Lac Z, Lac Y and Lac A are the structural genes which code the enzymes namely, ?-galactosidase, lactose permease and lactose transactylase (which are essential for lactose utilization); T denotes the terminater which stops the mRNA synthesis.

Under normal conditions, meaning, no lactose in the medium, the repressor blocks the operator of the lac operon and thereby no production of these enzymes. When lactose was introduced to medium transport to inside of the cell, act as inducer. The inducer will bind with repressor protein and lead to no more blockage of operator region. Then RNA polymerase starts synthesizing mRNA upto the terminator region. The transcription will happen until the presence of inducer (lactose). When lactose was exhausted in the medium, the repressor protein will again block the operator and no more mRNA synthesis takes place. This kind of gene regulation is referred as repression and sometimes induction type gene regulations are also common one.

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