The DNA has two important role in the cell, first is replication and the second is expression. Gene expression is accomplished by a series of events that contained in DNA is converted into molecule that take place in the cell. The information contained in DNA is converted into molecules that determine the mechanism of the cell. During the process of gene expression DNA is first copied into an mRNA molecule, which determines the amino acid sequence of molecule of protein. The process of synthesis of an RNA molecule corresponding to a gene is called transcription. By using base sequences and RNA molecule, proteins are synthesized in a definite order. Production of amino acid sequence from an mRNA base sequence is called translation. After completion of translation proteins are synthesized. Therefore, gene expression refers to protein synthesis through two major events, transcription and translation.
Gene regulation :
The DNA of a microbial cell consists of genes few to thousands, which do not express at the same time. At a particular time only a few genes express and synthesize the desired proteins. The other genes remain silent at this moment and express when required. Requirement of gene expression is governed by the environment in which they grow. This shows that the genes have a property to switch on and switch off.
Twenty different amino acids constitute different proteins. All are synthesized by codons. Therefore, synthesis of all amino acids require energy, which is useless because all the amino acids constituting proteins are not needed at a time. Hence, there is need to control the synthesis of those amino acids which are bot required.
Gene regulation :
The DNA of a microbial cell consists of genes few to thousands, which do not express at the same time. At a particular time only a few genes express and synthesize the desired proteins. The other genes remain silent at this moment and express when required. Requirement of gene expression is governed by the environment in which they grow. This shows that the genes have a property to switch on and switch off.
Twenty different amino acids constitute different proteins. All are synthesized by codons. Therefore, synthesis of all amino acids require energy, which is useless because all the amino acids constituting proteins are not needed at a time. Hence, there is need to control the synthesis of those amino acids which are bot required.
1. Lactose or Lac operon model :
The operon is the coordinated unit of genetic expression in bacterium. The concept of operon was introduced by Jacob and Monod in 1961, based on their observations on the regulation of lactose metabolism is E. coli. This is popularly known as lac operon.
In E. coli two proteins are necessary for the metabolism of lactose. These proteins are the enzymes β-galactosidase, which cleaves lactose to yield galactose and glucose and a carrier molecule galactosede permease, which is required for the entry of lactose into the cell.
The lac operon consists of a regulatory gene, operator gene (O) and three structural genes (Z,Y and A). There is a promoter site (P), where the enzyme RNA polymerase binds. The structural genes Z, Y and A respectively, code for the enzymes β-galactosidase, galactoside permease and galactoside acetylase.
The structural genes Z, Y and A transcribe into a single lage mRNA with 3 independent translation units for the sysnthesis of 3 different enzymes. An mRNA coding for more than one protein is known as polycistronic mRNA. Prokaryotic organisms contain a large number of polycistronic mRNAs.
The repressor (regulator) gene :
Repressor gene determines the transcription of structural genes. It is of two types : active and inactive repressor. It codes for amino acid of a defined repressor protein. After synthesis the repressor molecules are diffused from the ribose and bind to the operator in the absence of an inducer. Finally the path of RNA polymerase is blocked and mRNA is not transcribed. Consequently no protein synthesis occurs. This type of mechanism occurs in the inducible system of acive repressor.
Morover, when an inducer is present it binds to repressor protiens and forms an inducer-repressor complex. This complex can not bind to the operator. Due to formation of complex the repressor undergoes changes in conformation of shape and becomes inactive. Consequently the structural genes can synthesize the polycistronic mRNAs and the later synthesizes enzymes (Protiens).
Jacob and Monod could not identify the repressor protein. Gilbert and Muller-Hill (1996) succeeded in isolating the lac repressor from the lac mutant cells of E. coli inside which the lac repressor was about ten times greater than the normal cell. The lac repressor proteins have been crystallized. It has molecular weight of about 1,50,000 daltons. It consists of four subunits each has 347 amino acid residues and molecular weight of about 40,000 daltons.
Structural genes :
The structural genes form one long polycistronic mRNA molecule. The number of structural gene corresponds to the number of proteins. Each structural gene is controlled independently and transcribe mRNA molecule separately. This depends on substrates to be utilized. For example, in the lac operon three structural genes (Z, Y and A) are associated with lactose utilization. β-galactose is the product of lac Z that cleaves β-1,4 linkage of lactose and release the free monosaccharides. This enzyme is a tetramer of four identical subunits each with molecular weight of 1,16,400. The enzyme permease (a product of lac Y) facilitates the lactose to enter inside the bacterium. Permease has molecular weight of 46,500. It is hydrophobic. The enzyme transacetylase (30,000 molecular weight) is a product of lac A whose no definite role has beeb assigned.
The operator gene :
The operator gene is about 28bp in length present adjancent to lac Z gene. The base pairs in the operator region are palindrome. The operator overlaps the promoter region. The lac repressor protiens bind to the lac operator in vitro and protect part of the promoter region from the digestion of DNase. The repressor proteins bind to the operator and form an operator-repressor complex which is in turn physically blocks the translation of Z, Y and A genes by preventing the release of RNA plumerase to begin transcription.
The promoter gene:
The promoter gene is about 100 nucleotide long and continuous with the operator gene. The promoter gene lies between the operator gene and regulator gene. Like operators, the promoter region consists of palindromic sequence of nucleotides.
