1. Escherichia coli lac operon in the absence of lactose (disregard glucose)
When no lactose is present in E. coli lac I, regulator gene, is transcribed into mRNA and then translated into a repressor protein. Due to the lack of lactose, the repressor protein binds to the lac O operon site, operator, which blocks the promoter site (lac P) and structural genes (lac Z, lac Y, lac A). The blockage of these genes doesn’t allow the RNA polymerase to proceed, thus stopping any glucose and galactose from being derived from lactose (which in this case is absent).
2. Escherichia coli lac operon in the presence of lactose (disregard glucose)
When lactose is present in the cell, the regulation can proceed as usual. First lac I transcribes mRNA and then is translated into a repressor protein. However, the structural genes, lac Z, lac Y, and lac A, are able to make β-Galactosidase, Permease, and Transacetylase respectively. Then take the lactose and break it down into Glucose and Galactose, while also using β-Galactosidase to turn it into Allolactose which can bind to the repressor protein. When allolactose binds to the repressor it inactivates it, which allows for the RNA polymerase to move without hindrance completing the whole sequence.
3. Escherichia coli lac operon in the absence of glucose (disregard lactose)
When Glucose is absent adenosine triphosphate (ATP) is changed into cyclic AMP (cAMP) by adenylyl cyclase. Then cAMP molecules are joined together with cAMP receptor proteins (CRP) which as a unit bind to lac P and lac O to allow the sequence to continue.
4. Escherichia coli lac operon in the absence of glucose and lactose
When glucose and lactose are absent there is no energy to be derived to fuel the reaction. The Catabolite activator protein (CAP) is bound on the lac I site with cAMP, which is in abundance due to the lack of glucose. However, because no lactose is present the repressor protein is active, and the RNA polymerase is prevented from proceeding.
5. Describe where in the process of gene expression (transcription, post-transcription, translation, post-translation) this regulation takes place.
Transcription is the level at which gene expression is regulated. With the lack or absence of particular substances it is transcription first that is impacted and when it cannot occur none of the other processes can occur either. This means that post-transcription, translation, and post-translation are affected, but not the main site of the gene expression regulation.
By: Joy Brewer
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