Gene expression

Gene expression

The mechanism at the molecular level by which a gene is able to express itself in the phenotype of an organism is known as Gene expression. The mechanism of gene expression consists of biochemical genetics. It plays important role for the synthesis of specific RNAs and polypeptides. Transcription is defined as the formation of RNAs from genes (DNA). The information to the ribosome is carried by the transcribed mRNA. It translates it into an amino acid sequence of a polypeptide with the help of tRNA ( the process called translation). Peptides express the gene in the form of protein, enzyme or biochemical by forming a structural protein.

Regulation of gene expression controls over the functioning of genes. It is found by E. coli. Regulation of gene expression is of three types- they are inducible, constitutive and repressible. Regulation can be under positive or negative control.

1. Inducible:The regulation which is switched on in response to the presence of substrate is known as Inducible.
2. Constitutive: The regulation in which genes remain constantly expressing themselves is known as Constitutive.
3. Repressible: The regulation in which the product of gene activity, if already present, stops the activity of that gene is called Repressible.
4. Positive control: Under its control, the product of a regulatory gene activates expression of genes.
5. Negative control: Under its control, the product of a regulatory gene shuts off the expression of genes. Operons was found by Jacob and Monad (1961), that the genetic material possesses regulated gene units.

Operon concept

A genetic unit consisting of an operator, a promoter and one or more structural genes whose activity is influenced by a regulator is known as An operon. Jacob and Monad (1961) discovered Operon concept in prokaryote (e.coli). It is a group of coordinately regulated genes. it is also the products of which typically catalyze a multi-enzyme metabolic pathway and its controlling elements. Controlling elements contain the promoter, operator and regulator genes.

Despite, there are many operons in the bacterial cells, but the lactose or lac operon is the classic example of all operons which was discovered by Jacob and Monad.

Lactose (lac) operon

It is a typical example of an operon. The lactose metabolism in e. Coli was deeply studied by Jacob and Monad.Lactose is divided to glucose and galactose by three enzymes namely -galactosidase permease and thiogalactoside transacetylase which are produced by the structural genes via; Z,Y and A respectively.

Structure of the lac operon

The two classes of genes are needed to make a functional lac operon:

1. Structural genes: The lac operon contains three genes named Z, Y and A that code for three enzymes mentioned below:

• The gene that codes for -galactosidase is known as Z gene.
• The gene that codes for galactosidase permease is known as Y gene.
• The gene that codes for thiogalactoside transacetylase is known as A gene.

These genes are situated in a row adjacent with each other so that they are called linked. They are known as polycistronic. Operator and promoter genes are regulated by Structural genes.

• Operator genes:The genes which lies between the promoter and the structural genes is known as Operator genes. It acts as a switch. Whether operator gene is on or off depends if structural genes are expressed or not expressed. All the three structural genes are regulated by a single operator gene.
• Promoter gene: Proper initiation of transcription is directed by a single promoter gene. The lac Z, lac Y, and lac A genes are expressed as a polycistronic message from a common promoter. The binding of DNA-dependent RNA polymerase and promoter initiates the transcription of structural genes.

2. Regulatory gene: It is located away from structural genes. Hence it is often not considerable as part of the operon. However, it is the key element of an operon. It codes for the product that regulates the level of expression of structural genes. The regulatory gene constantly transcribes mRNA to produce the repressor protein.

Regulation of lac operon expression

The absence and presence of lactose (inducer) switch on or off the transcription of mRNA and protein synthesis. This phenomenon can be described in following steps:

1. When e. Coli is grown in a medium in the absence of lactose, the regulator gene produces a repressor protein that binds the operator gene and blocks its activity. RNA polymerase cannot move from promoter to structural genes. It stops the transcription of mRNA from structural genes and thus protein synthesis is switched off. Hence no enzymes are produced.
2. When the lactose is introduced in the medium, lactose binds to the repressor protein. In this way, repressor protein fails to bind to the operator gene. Then the operator gene remains active and the hence switch is turned on. It induces RNA polymerase to bind to promoter gene. RNA polymerase starts transcription of structural genes. mRNAs corresponding to all three enzymes are synthesized which get translated to produce three enzymes; Z gene codes for -galactosidase, Y gene for galactoside permease and A gene for thiogalactoside transacetylase. With the expression of these three enzymes, metabolism of lactose begins.

Synthesis of enzymes is continued unless and until all the lactose molecules are consumed. When the last molecule of lactose bound to repressor is consumed, the inactive repressor becomes active and thus binds to operator site to switch off the operon as normal.

Role of repression and constitutive enzymes

When a substrate required by the bacterium is supplied in an excess amount from outside, bacterium stops or inhibits the production of the substance. In another way, we can say that the gene is being inactivated. These inactivated genes are thus called repressible gene and the phenomenon is called repression.

However, some of the cellular activities are functioning normally and constantly such as glycolysis. The genes, that constantly expressed to take care of normal cellular activity such as glycolysis, are known as constitutive genes. The expressions of these genes are not regulated. The enzymes produced by bacterium for above function are known as constitutive enzymes. The constitutive enzymes are dehydrogenases.

Gene expressions in eukaryotes

The genome of higher eukaryotes is very complex. Eukaryotes genome contains DNA many times as compared prokaryotic genome. For example, drosophila has 5,000 to 10,000 genes. Human haploid genome seems to have at least 23,000 to 1,00,000 genes. In eukaryotes, most of the DNA is non-functional or inactive and known as excess DNA or repetitive DNA. The diploid organism has two sets of chromosomes. The genome in eukaryotes controls various functions such as; growth and division of cells, differentiation, and specialization of tissues such as muscles, liver, or heart in animals and parenchyma, chlorenchyma, xylem or phloem on plants. As the eukaryotes genome is very large, the gene expression and its regulation become very complex.

Significance of gene regulation

The significance of mechanism of gene regulation is as follows:

1. Gene regulation allows the metabolism of specific chemicals by a particular cell.
2. Gene regulation plays the important role in growth and differentiation causing morphogenesis.
3. It permits the cells to adjust to environmental changes.
4. The regulation of gene expression allows for the expression of only genes which are of immediate need of the cell.
5. Gene regulation helps the production of specific chemicals by specific cells.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.