#biologyanimation #lacoperon #operon #generegulation
Bacteria adapt to changes in their surroundings by using regulatory proteins to turn groups of genes on and off in response to various environmental signals.
Francois Jacob and Jacques Monod received the Nobel Prize for their experiments which increased our basic understanding of how the lactose metabolizing genes are regulated in E. coli.
There are three structural (protein-coding) genes involved in lactose metabolism in E. coli.
These 3 lac genes are organized into the lac operon.
An operon is a cluster of genes along with an adjacent promoter and operator that control the transcription of those genes.
When the structural genes in an operon are transcribed, a single mRNA is produced.
This mRNA is said to be polycistronic, because it carries the information for more than one type of protein.
lacz+ encodes beta galactosidase which breaks down lactose into glucose and galactose. lacY+
encodes lactose permease which transports lactose into the cell.
lacA+ encodes transacetylase whose function is not fully understood.
The operator (laco+) is a short region of DNA that lies partially within the promoter and that interacts with a regulatory protein that controls the transcription of the operon.
The regulatory gene lacI+ produces an mRNA from which is synthesized a repressor protein, that can
bind to the operator of the lac operon.
The general term for the product of a regulatory gene is a regulatory protein.
The lac regulatory protein is called a repressor because it keeps RNA polymerase from transcribing the structural genes.
In the absence of lactose, the lac repressor binds to the operator and keeps RNA polymerase from transcribing the lac genes.
When lactose is present, small amounts of it are converted to an isomer called allolactose which acts as an inducer to turn on the lac genes.
The lac genes are expressed because allolactose binds to the lac repressor protein, changing its shape so that it cannot bind to the lac operator.
RNA polymerase can then bind to the promoter and transcribe the lac genes.
Jacob and Monod produced mutations in the lac operon to show how they may affect the regulation of gene expression.
They produced a mutation in the lacI gene (lacI-) such that mutant, inactive repressor proteins were synthesized.
These proteins cannot bind to the operator.
The result is that the structural genes are expressed constitutively, that is, in the presence or
absence of lactose.
Mutations in the operator are called constitutive (lacOc).
DNA base-pair alterations in the operator region make this sequence unrecognizable to the repressor protein. Since the lac repressor cannot bind, the structural genes are constitutively (always) expressed in the absence or presence of lactose.
Another mutation in the lacI gene called lacls (superrepressor) showed no synthesis of the lac enzymes in the presence or absence of lactose.
This mutant repressor protein binds to the operator but is unable to recognize allolactose.
Therefore, the lac repressor binds to the operator even in the presence of allolactose, and transcription does not occur.
The lac operon is one example of how bacteria can turn on or turn off genes in response to environmental conditions.
The presence of lactose induces the synthesis of enzymes necessary to convert lactose into glucose. Mutations in this operon demonstrate how the different regions are controlled.

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