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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.
A transcriptional activator is a protein (transcription factor) that increases transcription of a gene or set of genes. [1] Activators are considered to have positive control over gene expression, as they function to promote gene transcription and, in some cases, are required for the transcription of genes to occur.
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.
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]
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.
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.
upregulation, activation, or promotion – increase the rate of gene transcription downregulation , repression , or suppression – decrease the rate of gene transcription coactivator – a protein (or a small molecule) that works with transcription factors to increase the rate of gene transcription
The activated form of RecA interacts with the LexA repressor to facilitate the LexA repressor's self-cleavage from the operator. [7] [8] Once the pool of LexA decreases, repression of the SOS genes goes down according to the level of LexA affinity for the SOS boxes. [7] Operators that bind LexA weakly are the first to be fully expressed.