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Activator binds to an inducer and the complex binds to the activation sequence and activates target gene. [2] Removing the inducer stops transcription. [2] Because a small inducer molecule is required, the increased expression of the target gene is called induction. [2] The lactose operon is one example of an inducible system. [2]
A typical operon. In genetics, an operon is a functioning unit of DNA containing a cluster of genes under the control of a single promoter. [1] The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo splicing to create monocistronic mRNAs that are translated separately, i.e. several strands of mRNA that each encode a single gene product.
The gab operon comprises three structural genes – gabD, gabT and gabP – that encode for a succinate semialdehyde dehydrogenase, GABA transaminase and a GABA permease respectively. There is a regulatory gene csiR, downstream of the operon, that codes for a putative transcriptional repressor [1] and is activated when nitrogen is limiting.
The lac operon in the prokaryote E. coli consists of genes that produce enzymes to break down lactose. Its operon is an example of a prokaryotic silencer. The three functional genes in this operon are lacZ, lacY, and lacA. [6] The repressor gene, lacI, will produce the repressor protein LacI which is under allosteric regulation.
Mutations which knocked out the trp R (repressor) gene still showed some regulation of the trp operon (these mutants were not fully induced/repressed by tryptophan). The total range of trp operon regulation is about 700 X (on/off). When the trp repressor was knocked out, one still got about 10 X regulation by the absence or presence of trp.
Toggle switch which operates using two mutually inhibitory genes, each promoter is inhibited by the repressor that is transcribed by the opposing promoter. Toggle switch design: Inducer 1 inactivates repressor 1, which means repressor 2 is produced. Repressor 2, in turn, stops transcription of the repressor 1 gene and the reporter gene. [14]
If the repressor has a higher affinity for its motif than the activator, transcription would be effectively blocked in the presence of the repressor. Tight regulatory control is achieved by the highly dynamic nature of transcription factors. Again, many different mechanisms exist to control whether a transcription factor is active.
In prokaryotes, a well-known activator protein is the catabolite activator protein (CAP), involved in positive control of the lac operon. In the regulation of gene expression , studied in evolutionary developmental biology (evo-devo), both activators and repressors play important roles.