Lac Operon Model
Lac Operon Model
The mechanism of Lac Operon was first described by F.Jacob and J. Monad in 1961.
For this work they were awarded by Nobel Prize in 1965.According to this model, an operon was defined as a ' unit of co-ordinate control of protein synthesis which consisted of an operator gene and structural gene. The operator gene controls the activity of number of structural genes, while structural gene produce the protein by transcribing mRNA. The regulator gene is not a part of operon but it controls the operator gene by producing repressor molecule.
Lac operon or Lactose operon in E.coli: It is an operon model proposed by F.Jacob and J.Monad to study the gene regulation mechanism in E.coli.
The mechanism of Lac operon and it's different units are described below:
1) Structural Gene: These gene's are directly associated withe the synthesis of cellular proteins.
These Gene's determine the primary structure of an individual protein. A lac operon contains three structural genes or cistrons, viz z, y, a. The products of these genes are involved in the breakdown of the Lactose. Gene Z contains 3063 base-pair and codes for an enzyme beta -galactosidase which cleaves lactose into glucose and galactose. Gene y contains 500 base pair and determine the structure of enzyme Galactosidase permease. This is a plasma membrane bound protein and facilitates the entrance of lactose into the cell. Gene consists of 800 base pair and codes for an enzyme called as thio galactosidase acetylase.This enzyme helps in transfer of acetyl group from acetyl-Coa to Galactosidase.These three genes are present in order of z- y- a
All the three enzymes are synthesized only when lactose is present.Thus lactose act to 'induce' the production of this enzyme and hence in this case the substrate act as an Inducer.
Operator gene: The unit of Operon concerned with the transcription of mRNAs is called the operator gene.It is situated close to the structural gene whose expression it regulates.It is the site of action for an activator or repressor that is it decide whether the transcription should be initiated or not. The operator gene is recognised buy repressor protein which binds to the operator forming an 'operator repressor Complex'.Transcription of the structural gene cannot occur when the repressor is bound to the promoter.It prevents the polymerase from binding the promoter site. the operator gene is usually located adjacent to the structural gene region of the gene.It is necessary to have operator genes in close linkage for function.This Gene regulate the formation of mRNA of the structural gene through transcription.
In E-Coli the operator element is found to be situated immediately adjacent to gene Z so that the overall gene order is O-z- y- a This operator consists of 26 base pair.
3) Regulator gene:The sequence that governs the rate of particular Protein synthesis in a given intracellular environment is called as an 'inducer'(i) or a 'regulatory gene'. Cheat codes for a protein which is 360 amino acids in length. The protein may be either an active repressor or an inactive repressor. The active form of the lac repressor however is a tetramer that contains 4 copies of one gene product the regulatory gene is closely linked to the Operon but is separated from that of the operator thereby the gene order is a dash dash Z y a this protein has 1 active site for operator and the other active site for inducer the gene i is 1111 base pairs long and is transcribed separately from the Gene of the Lac Operon as a monocistronic mRNA which is translated on the ribosome into a repressor protein. In the absent of the inducer, the Repressor binds to the lac operator sequence.It frequent RNA polymerase from binding to the promoter and transcribing the structural gene. This result in inhibition of the protein synthesis in presence of an inducer, the repressor protein binds to the inducer to form an "inducer repressor complex" and result in initiation of protein synthesis. The mutation of regulator gene will affect the rate of synthesis of polypeptide determined by the structural Gene but will not alter the structure of protein.
4) Promoter gene: The promoter region represent a sequence of bases which act as and actual site of transcription initiation. Operator Gene is the site of action of Repressor while promoter Gene is the actual site of transcription initiation. It is present upstream of operator gene. The initiation properties of promoter region can be modified by temporary changes in the intracellular environment. The promoter can be regulated by certain proteins called as regulatory protein or repressor. In presence of this proteins initiation of transcription by a regulated promoter is blocked.It is stimulated only by the interaction of other regulated promoter with other regulator molecules.
The repressor when attached to operator prevent the movement of RNA polymerase. According to Pribnow (1975), the promoter gene consists of three element or site and is constant in position with respect to each other: This sites are A) catabolite gene Activator B)initial binding site C) operator or mRNA initiation site.
The detailed study of these three sites has a revealed that operator gene is a part of promoter gene. Recently the upstream and downstream DNA sequences in many Operon have been worked out in detail.It is now revealed that the expression of gene of a particular Operon is regulated at several region including - 10, - 35 ,-60 nucleotides (minus sign indicate preceding the start point). The region at -10 is an important sequence and is called pribnow box and was discovered by pribnow in 1975 any changes in the upstream sequence of DNA have been found to affect drastically the synthesis of protein .
5) Inducer or effector molecule:
Inducer are effective is a small molecule it maybe sugar amino acid or a nucleoside that can bind to the regulator protein to change its ability to interact with an operator or promoter gene.Jacob and Monad suggested that a regulator protein will will bind to an operator and inhibit Operon expression in the absence of effector or inducer. But,when effector or inducer molecule appear in the intracellular environment, it will bind to the regulator protein and finally pull the regulator away from the operator. It result in the expression of the Operon.
The mechanism of Lac Operon was first described by F.Jacob and J. Monad in 1961.
For this work they were awarded by Nobel Prize in 1965.According to this model, an operon was defined as a ' unit of co-ordinate control of protein synthesis which consisted of an operator gene and structural gene. The operator gene controls the activity of number of structural genes, while structural gene produce the protein by transcribing mRNA. The regulator gene is not a part of operon but it controls the operator gene by producing repressor molecule.
Lac operon or Lactose operon in E.coli: It is an operon model proposed by F.Jacob and J.Monad to study the gene regulation mechanism in E.coli.
The mechanism of Lac operon and it's different units are described below:
1) Structural Gene: These gene's are directly associated withe the synthesis of cellular proteins.
These Gene's determine the primary structure of an individual protein. A lac operon contains three structural genes or cistrons, viz z, y, a. The products of these genes are involved in the breakdown of the Lactose. Gene Z contains 3063 base-pair and codes for an enzyme beta -galactosidase which cleaves lactose into glucose and galactose. Gene y contains 500 base pair and determine the structure of enzyme Galactosidase permease. This is a plasma membrane bound protein and facilitates the entrance of lactose into the cell. Gene consists of 800 base pair and codes for an enzyme called as thio galactosidase acetylase.This enzyme helps in transfer of acetyl group from acetyl-Coa to Galactosidase.These three genes are present in order of z- y- a
All the three enzymes are synthesized only when lactose is present.Thus lactose act to 'induce' the production of this enzyme and hence in this case the substrate act as an Inducer.
Operator gene: The unit of Operon concerned with the transcription of mRNAs is called the operator gene.It is situated close to the structural gene whose expression it regulates.It is the site of action for an activator or repressor that is it decide whether the transcription should be initiated or not. The operator gene is recognised buy repressor protein which binds to the operator forming an 'operator repressor Complex'.Transcription of the structural gene cannot occur when the repressor is bound to the promoter.It prevents the polymerase from binding the promoter site. the operator gene is usually located adjacent to the structural gene region of the gene.It is necessary to have operator genes in close linkage for function.This Gene regulate the formation of mRNA of the structural gene through transcription.
In E-Coli the operator element is found to be situated immediately adjacent to gene Z so that the overall gene order is O-z- y- a This operator consists of 26 base pair.
3) Regulator gene:The sequence that governs the rate of particular Protein synthesis in a given intracellular environment is called as an 'inducer'(i) or a 'regulatory gene'. Cheat codes for a protein which is 360 amino acids in length. The protein may be either an active repressor or an inactive repressor. The active form of the lac repressor however is a tetramer that contains 4 copies of one gene product the regulatory gene is closely linked to the Operon but is separated from that of the operator thereby the gene order is a dash dash Z y a this protein has 1 active site for operator and the other active site for inducer the gene i is 1111 base pairs long and is transcribed separately from the Gene of the Lac Operon as a monocistronic mRNA which is translated on the ribosome into a repressor protein. In the absent of the inducer, the Repressor binds to the lac operator sequence.It frequent RNA polymerase from binding to the promoter and transcribing the structural gene. This result in inhibition of the protein synthesis in presence of an inducer, the repressor protein binds to the inducer to form an "inducer repressor complex" and result in initiation of protein synthesis. The mutation of regulator gene will affect the rate of synthesis of polypeptide determined by the structural Gene but will not alter the structure of protein.
4) Promoter gene: The promoter region represent a sequence of bases which act as and actual site of transcription initiation. Operator Gene is the site of action of Repressor while promoter Gene is the actual site of transcription initiation. It is present upstream of operator gene. The initiation properties of promoter region can be modified by temporary changes in the intracellular environment. The promoter can be regulated by certain proteins called as regulatory protein or repressor. In presence of this proteins initiation of transcription by a regulated promoter is blocked.It is stimulated only by the interaction of other regulated promoter with other regulator molecules.
The repressor when attached to operator prevent the movement of RNA polymerase. According to Pribnow (1975), the promoter gene consists of three element or site and is constant in position with respect to each other: This sites are A) catabolite gene Activator B)initial binding site C) operator or mRNA initiation site.
The detailed study of these three sites has a revealed that operator gene is a part of promoter gene. Recently the upstream and downstream DNA sequences in many Operon have been worked out in detail.It is now revealed that the expression of gene of a particular Operon is regulated at several region including - 10, - 35 ,-60 nucleotides (minus sign indicate preceding the start point). The region at -10 is an important sequence and is called pribnow box and was discovered by pribnow in 1975 any changes in the upstream sequence of DNA have been found to affect drastically the synthesis of protein .
5) Inducer or effector molecule:
Inducer are effective is a small molecule it maybe sugar amino acid or a nucleoside that can bind to the regulator protein to change its ability to interact with an operator or promoter gene.Jacob and Monad suggested that a regulator protein will will bind to an operator and inhibit Operon expression in the absence of effector or inducer. But,when effector or inducer molecule appear in the intracellular environment, it will bind to the regulator protein and finally pull the regulator away from the operator. It result in the expression of the Operon.
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