Most bacterial gene regulations occur at the transcriptional level because it would be a waste to make the RNA, if RNA and its encoded protein are not needed. Regulation in prokaryotes was explained through Operon Model by Jacob and Monad.
In E.coli two proteins are necessary for the metabolism of lactose sugar. ß-galactosidase, cleaves lactose to yield galactose and glucose and a carrier molecule, galactoside permease, which is required for the entry of lactose into the cell. If a culture of E.coli (Genotype- lac+ ) is growing in a medium lacking lactose, the intracellular concentration of ß-galactosidase is almost negligible. On the other hand if lactose is present in the growth medium the concentration of these proteins is raised. If lactose is added to a lac + culture growing in a lactose free medium, both ßgalactosidase and permease are synthesised simultaneously. These facts led to the view that the lactose (lac) system is inducible and the lactose is inducer. Regulation of the lac system is explained by operon model, which has the following features
- Products of the z and y genes are encoded in a single polycistronic mRNA molecule.
- The promoter for this mRNA is immediately adjacent to the o region. Promotor mutations (p- ) completely incapable of making both both ßgalactosidase and permease have been isolated and located between i and o.
- The operator is a sequence of bases to which the repressor protein binds.
- When the repressor protein is bound to the operator, transcription of lac mRNA cannot be initiated.
- Inducers stimulate mRNA synthesis by binding to the repressor. This binding alters the three-dimensional structure of the repressor so it cannot bind to the operator.
Thus in the presence of an inducer the operator is unoccupied and the promoter is available for initiation of mRNA synthesis. This is often called derepression. If lactose and glucose are added to a culture of wild type E. Coli cells, the lac operon is not induced. This effect of glucose is the result of a second regulatory mechanism known as catabolic repression because in this situation bacteria has the choice of glucose to metabolize in place of lactose hence there is no use of expression of lacoperon.
Tryptophan (trp) operon is responsible for the synthesis of tryptophan. Regulation of this operon is based on the principle that when tryptophan is present in growth medium, there is no need to activate the trp operon. Thus there is a regulatory mechanism that turns trp transcription off when adequate tryptophan is present and turns it on when it tryptophan is absent. This operon is active in repression rather than the induction.
Tryptophan is synthesized in five steps, each requiring a particular enzyme. The genes encoding these enzymes are adjacent to one another in the sane order as their use in biosynthesis pathway. They are translated from a single polycistronic mRNA and are called trpE, trpD, trpC. trpB and trpA. The trpE gene is the first one translated. Adjacent to this gene are the promoter, the operator and two regions called leader and attenuator, which are designated as trpL and trp a (Not trpA). The repressor gene trpR is located very far from this gene cluster.
The protein product of the trpR gene, which is often called as trp aporepressor, does not bind to the operator unless tryptophan is present. The aporepressor protein and tryptophan molecule join together to form an active repressor that binds to form an active repressor that binds to the operator.
Aporepressor alone – No repressor ——–> (Transcription occurs)
Aporepressor + Tryptophan ———–> Active Repressor + Operator —————-> Inactive promoter (Transcription does not occur)
Only when tryptophan is present, an active repressor molecule inhibits transcription. This repressor-operator mechanism is sufficient on-off switch for trp operon, however an additional mechanism allows a fine control in which the enzyme concentration is varied according to the amino acid concentration. These are
- Premature termination of transcription before the first structural gene is reached (Attenuation)
- Regulation of the frequency of this termination by the concentration of amino acid.