Some genes are always transcribed--constitutive or housekeeping genes. Some genes are only transcribed when needed--transcriptional regulation.

The classic example of bacterial gene regulation [local]--the lactose operon [local]

1. Lactose permease (or galactoside permease). It transports lactose into the cell. It is a product of the Y gene.
2. Beta-galactosidase. It hydrolyzes lactose into galactose (Gal) and glucose (Glu). It is a product of the Z gene.
3. Beta-galactosidase can also cartalyze the transformation of lactose {Galß(1-->4)Glu} to allolactose {Gal-ß(1-->6)Glu}.
4. The phosphotransferase sytem (PTS) transports glucose into the cell. Glucose is phosphorylated during the transfer using PEP as the phosphoryl donor.
5. Adenylate cyclase. It is turned on when phosphorylated by PTS.
6. Phosphodiesterase hydrolyzes cAMP and lowers its concentration in the cell.
7. CRP site. CRP (cAMP receptor protein) binds to cAMP and undergoes a conformational change allowing it to bind to the CRP site and activate the promoter.
8. The promoter to which the RNA polymerase binds, in the absence of the repressor.
9. The R-site. The repressor protein binds to the R-site when allolactose (the inducer) is absent. (The R-site is also referred to as the operator.

Glucose is the primary source of energy. Any other sugar is metabolized only when glucose is exhausted or cannot be used for some other reasons.

When glucose and lactose are available in the growth medium.

• Glucose is transported into the cell by PTS.
• The PTS prevents the transport of lactose into the cell.
• The PTS does not stimulate adenylate cyclase, consequently levels of cAMP are low.
• CRP-cAMP complex is not formed.
• the repressor protein remains bound to the R-site. The lactose operon is repressed. Only 1 or 2 molecules of beta-galactosidase are present in the cell (basal levels).
• Cell consumes glucose exclusively.

When glucose is consumed:

• A small amount of lactose leaks into the cell.
• The few molecules of beta-galactosidase already present catalyze the hydrolysis of lactose.
• Beta-galactosidase transforms some lactose into allolactose.
• Allolactose binds to repressor protein, which dissociates from the R-site
• Lack of external glucose causes PTS to phosphorylate adenylate cyclase, which is activated and produces cAMP.
• cAMP binds to CRP protein.
• CRP protein undergoes a conformational change and binds to the CRP-site near the promoter.
• This activates the promoter to bind RNA polymerase, which can now proceed past the R-site and begin RNA synthesis.
• The rate at which RNA polymerase initiates transcription increases.
• Rate of mRNA synthesis for Z and y proteins increases dramatically.
• These mRNAs for Z and Y genes are translated to form beta-galactosidase and lactose permease, respectively.
• Lactose starts to be catabolized.
• All this happens in less than a minute.

Animation [local] of the lactose operon.

Other examples of regulation in bacteria include the arabinose operon [local] and the tryptophan operon . Transcriptional regulation in eukaryotes is more complex, involving more transcription factors, response elements [local], sequence motifs and enhancers, as well as mRNA processing and turnover.

Quiz 1M	Quiz 2M	Quiz 3M	Quiz 4C	Quiz 5C
Quiz 6E

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