Mar 20, 2025 Transcriptional Regulation in Prokaryotes & Eukaryotes

Bacterial Gene Regulation

• E coli “eat” lactose sugar (break it down for energy)

• If lactose is present in the environment, E. coli turn on genes that code for proteins that (1) allow it to cross the membrane and enter the cell and (2) break it down into glucose and galactose (which eventually broken down further to extract energy) 

• If lactose is not present in the environment, E. coli keep these lactose metabolism genes in the off state to avoid wasting energy making proteins it doesn’t need

  • The genes encoding the proteins that import and break down lactose are located in the lac operon 

• Operon: Region in bacterial genome that contains multiple genes under the control of a single promoter (i.e., they’re regulated together) 

  • The genes included in a given operon usually code for proteins involved in the same cellular process 

Anatomy of the lac operon

• The lac operon contains 3 genes that code for 3 proteins involved in lactose metabolism

  • lacZ: codes for B-gal, the enzyme that breaks down lactose into glucose and galactose 

  • lacY: codes for lactose permease, a transmembrane protein that transports lactose into the cell

  • lacA: codes for another protein you don’t have to worry about 

    • One promoter (lacP) controls the transcription lacZ, lacY, and lacA

    • A repressor binding site called an “operator” (lacO), which binds a repressor encoded by lacl, located outside the operon

When lactose isn’t present 

• Cell’s don’t need lactose metabolism proteins 

• During these times, lacI is constitutively (continually, constantly) expressed 

• This produces repressor proteins that bind to lacO site

• When repressor is bound to lacO lac operon can’t be transcribed because RNA polymerase can’t bind to lacP site

  • Thus the transcription of the lac operon is REPRESSED 

When lactose is present 

• Repressor gets repressed, allowing for transcription (“expression”) of lac operon

• Lactose will bind to the repressor protein, which changes its shape

• This makes repressor unable to bind to lacO sequence 

• RNA polymerase can thus bind to promoter (lacP), lacZ, lacY and lacA can get transcribed, lactose can get metabolized

• Thus lactose indirectly “un”represses (unblocks) lac operon transcription 

• Note for transcription to proceed an activator protein must ALSO bind to the lac operon

Regulatory Factors 

• Each gene has its own regulatory sequences associated with it, which are recognized by specific TF proteins 

• What explains this specificity?

  • Different bases have different charges exposed in the DNA grooves 

  • A TF protein with a complementary shape and pattern of charges to a DNA sequence will “fit” the DNA sequence and be able to bind to it