lac operon quiz

• Lac operon


  A cluster of genes under the control of a single promoter
• 4 different growth conditions

  • Lactose
  • Glucose
• Transcription


  The process of copying genetic information from DNA into RNA
• lac repressor binds to operator

  Blocks RNA polymerase, preventing transcription even in the presence of cAMP CRP
• lac repressor binds to operator


  Blocks RNA polymerase. CRP does not bind, cAMP is low
• RNA polymerase binds to operon


  In absence of lac repressor. But with cAMP (CRP activation of RNA Poly), only low level of expression
• Operon


  A cluster of genes under the control of a single promoter
• Lac operon

  • Bacteria primarily use glucose as an energy source, but can utilize lactose as an alternative when glucose levels are low
• Beta-galactosidase


  Enzyme that hydrolyzes lactose into glucose and galactose, including its isomer, allolactose
• Galactosidase permease


  Facilitates the transport of lactose across the bacterial membrane
• Thiogalactoside transacetylase


  Enzyme that aids in the detoxification process
• Translation of the genes in the lac operon is independent
• Each gene in the lac operon is preceded by a ribosome binding site
• Default state of the lac operon

  The lac operon is usually in the "OL" state, meaning there is no transcription of the genes
• Regulatory gene (I)


  Codes for the lac repressor protein
• Lac repressor


  • Binds to the operator (O1), which is situated near the transcription start site, thereby preventing RNA polymerase from binding to the promoter
  • Is a tetramer formed by two tethered dimers
  • Has secondary binding sites (O2 or O3), which cause DNA looping, ultimately blocking the initiation of transcription
• Induction


  When glucose is scarce and lactose is present, allolactose or IPTG (Isopropyl β-D-1-thiogalactopyranoside) acts as an inducer
• Allolactose


  An isomer of lactose, binds to the lac repressor, inducing a conformational change that causes the repressor to dissociate from the DNA, allowing transcription to proceed
• Catabolite repression


  In the presence of glucose, catabolite repression restricts the expression of catabolic genes that produce enzymes to metabolize other sugars
• CRP-cAMP complex (CAP)


  When glucose is scarce, the CRP-cAMP complex (CAP) binds near the Lac promoter, facilitating the binding of RNA polymerase and enhancing transcription
• Lac promoter


  Exhibits weak expression in the presence of the CRP-cAMP complex. For high expression, lactose must be present, and glucose absent
• The lac operon is part of a regulatory network of operons controlled by a common regulator (e.g., CRP-cAMP)
• Trp operon


  Regulated by the Trp repressor, which is a homodimer
• Trp repressor


  When tryptophan is abundant, it binds to the Trp repressor, causing a conformational change that enables the repressor to bind to the operator, thereby inhibiting transcription
• Operator


  The operator sequence overlaps the promoter, blocking RNA polymerase from initiating transcription
• Leader sequence


  Plays a crucial role in attenuation, fine-tuning the expression of the trp operon
• Seq1


  Codes for two adjacent tryptophan residues, serving as an operon regulatory device
• Seq2


  Can bind to seq 8
• Seq3 and 4


  Rich in G-C pairs, form a stem-loop structure, acting as a translation terminator
• Riboswitch


  A regulatory segment of mRNA that binds to a small ligand. When the ligand binds, it causes a conformational change in the mRNA, inhibiting both transcription and translation
• Riboswitches are found in the untranslated region (UTR) of the 5' end of mRNA
• Riboswitch function


  Provides a feedback loop regulation mechanism