An operon system is a genetic regulatory system found in bacteria which regulates and expresses the bacterial gene. It allows proteins synthesis to be controlled coordinately in response to the needs of the cell and consists of structural genes, promoter and cis-acting regulatory sequences recognized by regulatory gene products. The operon hypothesis (or theory) was first proposed by the French microbiologists François Jacob and Jacques Monod in the early 1960s.
The operon model possesses three elements:
1. A set of structural genes: These genes encodes the proteins to be regulated.
2. An operator site: It is a DNA sequence that regulates transcription of the structural genes.
3. A regulator gene: It encodes a protein that recognizes the operator sequence.
Tryptophan operon (trp operon)
Tryptophan operon (trp operon) is the regulation of transcription of the gene responsible for biosynthesis of tryptophan. It contains five structural genes encoding enzymes for tryptophan biosynthesis with an upstream trp promoter (P trp) and trp operator sequence (O trp), these are:
1. Trp E: It encodes the enzymes Anthranilate synthase I.
2. Trp D: It encodes the enzyme Anthranilate synthase II.
3. Trp C: It encodes the enzyme N-5-phsphoribosyl anthranilate isomerase and indole-3-glyceerolphosphate synthase.
4. Trp B: It encodes the enzyme tryptophan synthase-B subunit.
5. Trp A: It encodes the enzyme tryptophan synthase-A subunit.
In the presence of tryptophan (switch off):
When tryptophan is present in the medium, the trp operon functions in the following manner:
- When tryptophan is present in the medium, the enzyme of biosynthesis of tryptophan ate not needed and so expression and regulation of this gene is turned off.
- This achieved by binding tryptophan to the inactive repressor and activates it, so that it binds to the operator region and stops transcription of these genes.
- Binding of repressor protein to operator hinder the promoter and RNA polymerase to initiate the process of transcription. Thus, transcription remains halted.
- Here inactive repressor is called as aporepressor and the tryptophan is called as co-repressor.
In the absence of tryptophan (switch on):
When tryptophan is absent in the medium, the trp operon functions in the following manner:
- When tryptophan is absent in the medium, the enzymes for biosynthesis of tryptophan are needed and so expression or regulation of these genes is turned on.
- This occurs when the inactive repressor (or aporepressor) does not find tryptophan (or co-repressor) to get bound to and become active.
- As a result, repressor protein does not get bind to the operator region of the operon. Hence there is no obstacles present in the operon to hinder the transcription process.
- Thus, RNA polymerase enzyme bind with promoter region of the operon and initiates the process of transcription of the structural genes of the operon.
Lactose operon (Lac operon)
The lactose operon (or lac operon) of Escherichia coli (or E. coli) is a group of three structural genes encoding proteins involved in lactose metabolism and the sites on the DNA involved in the regulation of the operon. It is one of the most studied operons in E. coli.
Structure of Lactose Operon (or Lac operon)
In lac operon, the three structural genes that codes for proteins involved in lactose metabolism are: lac Z, lac Y and lac A genes.
1. LacY gene: It encodes enzyme galactoside permease (also known as lactose permease) which transports lactose into the cell across the cell membrane.
2. LacZ gene: It encodes enzyme β-galactosidase which hydrolyses (or breaks down) lactose into glucose and galactose.
3. LacA gene: It encodes enzyme transacetylase which transfers an acetyl group from acetyl-CoA to β-galactosides.
> Transcription starts from a single promoter (P lac) that lies upstream of these structural genes and binds to RNA polymerase enzyme.
> An operator site (O lac) is also present between the promoter and the structural genes, and
> An inhibitory gene (or lac I gene) is present in the upstream of the operon that codes for the lac repressor protein.
> The lac I gene has its own promoter (PlacI) which binds to RNA polymerase enzyme and leads to transcription of lac repressor mRNA and hence the lac repressor protein monomers are produced.
> Four identical repressor monomers come together and forms active tetramer which binds tightly to the lac operator site (O lac).
> Allolactose acts as an inducer in lac operon, which is formed by converting lactose present in the medium. This allolactose binds to the repressor protein in the lac operon.
In the presence of lactose (switch on):
When lactose is present in the medium, the lac operon functions in the following manner:
- When lactose is present in the medium, the enzymes for consuming (or taking) it inside the cell are needed and so regulation and expression of gene is turned on.
- This occurs when lactose (in the medium) binds to the repressor protein, discouraging them to bind to the operator site of the lac operon.
- Hence, there are no obstacles present in the lac operon to hinder RNA polymerase enzyme, and so RNA polymerase enzyme binds to the promoter and initiates the process of transcription of the structural genes of the operon.
In the absence of lactose (switch off):
When lactose is absent in the medium, the lac operon functions in the following manner:
- When lactose is absent in the medium, the enzymes for consuming (or taking) it inside the cell are not needed and so regulation and expression of gene in turned off.
- This occurs when repressor proteins bind to the operator site of the lac operon.
- This hinders RNA polymerase enzyme to bind to the promoter and initiate the transcription process. Hence, the operon does not get transcribed.