Regulation of gene expression in prokaryotes and eukaryotes.

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Regulation of gene expression in prokaryotes and eukaryotes

For a bacterium, it would be wasteful to transcribe all its genes at the same time- for example, it would be silly for a bacterium to produce enzymes for the synthesis of an amino acid, if it was readily available from its environment.

And so bacterium have evolved a number of mechanisms for regulating gene expression. Potentially, there are 3 sites of control:

  • by changing gene transcription
  • by changing mRNA turnover time
  • by changing the rate of translation

The mechanisms that I wish to discuss are those involving the regulation of gene transcription.

Prokaryotic genomes differ to eukayotic genomes, in various manners, for example, eukaryotic genomes contain significantly more non-coding sequences than do those of prokaryotes. Also, many bacterial genes exist as operons, the genes are coordinately arranged to give proteins with related functions. Operons may be inducible, or repressible, and include the lac operon (an inducible operon, with genes coding for enzymes needed for lactose metabolism) and the trp operon (a repressible operon with genes coding for ezymes involved in the biosynthesis of tryptophan).

The lac operon

There are 3 genes in this operon (lac Z, Y and A), and, transcribed sequentially into a single mRNA strand, they code for enzymes needed by E.Coli for lactose metabolism. Although there is always a basal transcrption rate for this operon, in the presence of lactose, the transcription rate increase rapidly.

Upstream from the lac operon lies a sequence coding for a protein known as the lac repressor. This binds to the operon at a position known as the operator, which lies between the promoter and beginning of the lac Z gene, and thus inhibits lac operon transcription by blocking the path of the RNA polymerase.

In the presence of allolactose (a lactose isomer and metabolic intermediate), the repressor fails to bind to the operator, as the allolactose adheres to the repressor, causing a conformational change. Hence when lactose concentration is high, it induces the transcription of the operon. Once the substrate has all been metabolized, the repressor is able to bind to the operator once more, preventing unnecessary transcription of the genes.

Catabolite repression

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In addition to the regulatory system described above, there is another mechanism for ensuring optimal substrate metabolism. If glucose is present, bacteria will use this as a substrate of preference, and so the cell has a mechanism for switching off the lac operon, when glucose is available. This mechanism is called catabolite repression. Glucose inhibits adenyl cyclase, an enzyme that catalyzes ATP → cAMP. cAMP binds to the catabolite activator protein (CAP), and bound CAP promotes a higher rate of lac operon transcription. So if glucose levels are high, cAMP levels are low, and CAP doesn’t activate, so transcription rate of the ...

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