F.jacob and j.monad in 1961 on the basis of their study on the inducible operon system for the synthesis of beta- galactosidase enzyme in E.coli proposed a model in order to explain the induction or repression of enzyme synthesis, this model is popularly known as operon model. according to this model, an operon was defined as a unit of co-ordinated control of protein synthesis or an operon is a co-ordinated unit of gene expression which consists of an operator gene which controlled the activity of an number of structural genes which took part in the synthesis of protein, this means that the structural genes will synthesize mRNA under the operation control of an operator gene. The operator gene in its turn is under the control of a repressor molecule synthesized by a regulator gene. Which is not a part of the operon, thus the members of an operon are transcribed co-ordinately a single, long polycistronic m RNA molecule. There are mainly two types of operon in use.
The term ‘‘operon’’ was first proposed in a short paper in the Proceedings of the French Academy of Sciences in 1960 .From this paper, the so-called general theory of the operon was developed. This theory suggested that all genes are controlled by means of operons through a single feedback regulatory mechanism: repression. Later, it was discovered that the regulation of genes is a much more complicated process. Indeed, it is not possible to talk of a general regulatory mechanism, as there are many, and they vary from operon to operon. Despite modifications, the development of the operon concept is considered one of the landmark events in the history of .
Components of operon:
The structural genes:
The structural genes form a single long polycistronic m RNA molecule the number of structural genes corresponds to the number of proteins. each structural gene is controlled independently and transcribe mRNA molecule separately, this, depends on substrate to be utilized. Example: in lac operon three structural genes (Z, Y, A) are associated with lactose utilization beta-galactosidase is the product of lac Z that cleaves beta (1-4) linkage of lactose &releases the free monosaccharides.the enzyme permease (a product of lacy) facilitates the lactose the enter inside the bacterium. The enzyme transacylase is a product of lac A where no definite role has been assigned. The lac operon consists of a promoter (p) operator (o) together with structural genes .the lac operon cannot function in the presence of sugars other than lactose
The operator gene:
The operator gene is present adjacent to lacZ gene .the operator gene overlaps the promoter region .the lac repressor protein binds to the operator invitro& protect part of the promoter region from the digestion of DNase .the repressor protein binds to the operator &forms an operator –repressor complex which in turn physically black the transcription of Z, Y&A genes by preventing the release of RNA polymerase to begin transcription.
The promoter gene:
The promoter gene is long nucleotide &continuous with the operator gene. The promoter gene lie between the operator ®ulator gene, like operators the promoter region consists of palindromic sequences of nucleotides (i.e show 2 fold geometry from a point).these palindromic sequence are recognized by such proteins that have symmetrically arranged subunits. This section of two fold symmetry is present on the CRP site(c-AMP receptor protein site that binds to a protein called CRP).the CRP is encoded by CRP gene, it has been shown experimentally that CRP gene binds to cAMP (c AMP found in e.coli & other organisms) molecule & form a cAMP CRP complex. This complex is required for transcription because it binds to promoter& enhances the attachment of RNA polymerase to the promoter therefore it increases the transcription &translation process.
The repressor (regulator) gene:
Regulator gene determines the transcription of structural gene. It is of two types-active & inactive repressor. It codes for amino acids of a defined repressor protein. After synthesis, the repressor molecules are diffused from the ribosome & bind to the operator in the absence of an induces. Finally the path of RNA polymerase is blocked & m RNA is not transcribed consequently; no protein synthesis occurs .this type of mechanism occurs in inducible system of active repressor. Moreover when an inducer is present it binds to repressor proteins 7forms an inducer –repressor complex. Due to formation of complex the repressor undergoes changes in the confirmation of shape 7 becomes inactive consequently the structural genes can synthesize the polycistronic m RNA and later synthesize enzyme.
In contrast in the reversible system the regulator gene synthesis repressor protein that is inactive & therefore fails to binds to operator, consequently ,proteins are synthesized by the structural genes .however the repressor protein can be activated in the presence of an co-repressor. the co-repressor together with repressor proteins forms the repressor-co repressor complex. This complex binds to operator gene & blocks the protein synthesis
Types of operon:
1. Lactose (Lac) operon:
The regulatory mechanism of operon is responsible for the utilization of lactose as a carbon source that is why it is called as lac operon. the lactose utilizing system consists of 2 types of components i.e the structural genes (lacZ, lacy, lacA) the products of which are required for transport and metabolism of lactose ®ulatory genes (lacI, lacP, lacO).these two components together comprises of lac operon .one of the most key features is that operon provides a mechanism for the co-ordinated expression of structural genes controlled by regulatory genes. Operon shows polarity i.e. the genes Z, Y, A synthesize equally qualities of 3 enzymes beta-galactosidase by lac Z, permease by lac Y & acetylase by lac A. These are synthesized in an order i.e. beta-galactosidase at first and acetylase in the last.
Regulation of lac operon:
Regulation of the lac operon by repressor is called negative control. The lac operon is also under positive control by CRP (or cAMP Receptor Protein; also known as CAP or catabolite activator protein). CRP or CAP is now thought to be bound to its lac binding site at all times (even during repression). During induction, the inducer (either the natural inducer, allolactose, or the synthetic inducer, IPTG, binds to the lac repressor. Inducer-bound repressor does not bind to operator sites. This allows RNA polymerase to bind to the promoter and start transcribing the lac operon.
Negative (lac repressor) -------------Bound to DNA-----------Not bound to DNA
(Type of Control) (Operon off) (Operon on)
Positive(CRP protein) --------------- Bound to DNA-----------Not bound to DNA
(Type of control) (Operon on) (Operon off)
2. Tryptophan (Trp) operon:
The tryptophan operon of E.coli is responsible for the synthesis of the amino acids tryptophan regulation of this operon occurs in such a way that when tryptophan is present in the growth medium, Trp operon is not active but, when adequate trp is present, the transcription of the operon is inhibited, however when its supply is insufficient transcription occurs, the Trp is quite different from the lac operon in that trp acts directly in the repression system rather than as an inducer. Moreover since the trp operon encodes a set of bio-synthetic caranabolic rather than a catabolic enzyme neither glu nor c AMP –CAP has a role in the operon activity.
Regulation of Trp Operon:
Trp is synthesized in 5 steps each required a particular enzyme.in E.coli chromosome the genes encoding these enzymes are located adjacent to one another in the same order as they are used in the bio-synthetic pathway they are translated from a single polycistronic m RNA molecule. These genes are called TrpE, TrpP, TrpC, TrpB, TrpA, The TrpE gene is the first one translated. Adjacent to the Trp E gene are the promoter, the operator &2 region, called the leader and the attenuated which are designated as TrpL & TrpA respectively .the repressor gene TrpR is located quite for from the gene cluster .the regulatory protein of the repressor system o the TrpR operon is the product of the TrpR gene. mutations either in this gene or in the operator cause constitute initiation of transcription of Trp-m RNA on the lac operon .this regulatory protein is called Trp apo repressor &it does not bind to the operator ,unless Trp is present .the apo repressor &the tryptophan molecule joins together to form an active trp repressor which binds to the operator. The reaction scheme is as follows:
Apo repressor (no trp) ---------------active repressor (transcription occurs)
Apo repressor+trp------------------active repressor +operator---------------- inactive operator (transcription does not occur)