Ch. 13: Repression and Lac Operon

Question 1

Repression

Answer 1

 Repression inhibits gene expression
 A protein acts as the repressor by binding to the DNA and preventing transcription
 The repressor may bind only in the presence of a co-factor, which may itself be a product of the structural gene

Question 2

Enhancers/Activators

Answer 2

 Can cause expression to occur at a promoter by binding to the regulatory region
 Make promoter
sequence available to RNAP, or enhance binding to the sequence by sigma factor

Question 3

Bacterial Two-Hybrid

Question 4

induction

Answer 4

 Induction is the process by which a set of genes is turned on by the presence of an inducer
 In many cases, the inducer is a substrate for the gene system or operon
 An operon is defined as a set of genes that are contiguous in the genome and coordinately regulated
to encode some functional genes
- A repressor and inducer may work together to regulate the expression of the genes in an operon

Question 5

the lac operon

Answer 5

 Parts of the operon- structural genes lacZYA, regulatory lacI, promoter regions and operator region (site where repressor binds)
 When lactose is absent, repressor binds, prevents transcription
 When lactose is present, it binds to LacI, prevents it from binding DNA, allows transcription to be induced

Question 6

Catabolite repression

Answer 6

• Lactose can act to induce lac operon, but what if lactose and glucose are present?
   Glucose is the preferred substrate, and this is effected using catabolite repression
   The effector protein is CAP (crp gene),which binds cAMP,
  inversely proportional to glucose

Question 7

Arabinose Operon

Answer 7

• Uses both positive and negative control
   AraC protein is the repressor, dimer binds to O2 and I1
   When arabinose is present, binds in different conformation, allows txn (no loop)
   Also uses cAMP-CAP to increase transcription rate

Question 8

Repression of trp operon

Answer 8

 Trp operon acts to
synthesizes tryptophan
 In the presence of large amounts of tryptophan, a
repressor binds to the trp operon, repressing the synthesis system

Question 9

transcription attenuation

Answer 9

Trp codons in leader peptide slow down ribosome, weaker
secondary structure than if sequence 2 is blocked