Question

For Questions 4-18, consider the mutations described in the left-hand column below. Identify whether the Lac operon will be ON or OFF in each of the environments listed across the top. The expression of the normal (wild type) operon is given. Note that "ON" and "OFF" refer to whether the operon is expressed at high levels (ON) or basal//low/no levels (OFF). (Hint: mutations render the indicated site inactive while leaving the remaining part of the operon or protein functional.) Note that question numbers are indicated within each box. No Glucose or Lactose Mutation Lactose only Glucose +Lactose Wild type operon OFF OFF ON CAP site mutated Operator mutated CAP allosteric site mutated 10 13 14 15 Lac repressor allosteric site mutated Lac repressor DNA binding domain mutated 17 16 18 Use this information to answer questions 19-24. Assume that in a bacterial species you discover a cluster of three genes (A, B, and C) that account for the enzymatic production of a cellulose-like carbohydrate (clc) that is needed as a component of its cell wall. When there is sufficient clc is in the cell, the three enzymes are not synthesized. However, when more clc is needed, the three enzymes are abundantly produced. This cluster of genes appears to be under coordinate control because deletion of a distant gene (R) results in constitutive expression of all three genes even when excess clc is present. When mutations (P ) occur in a region just upstream of gene A, no synthesis of the three enzymes occurs even when clc levels are extremely low Mutations (O ) that occur right next to the P mutations result in constitutive expression of all three genes even when clc is present at high levels. 19. Do the three genes (A, B, and C) behave as if they were part of an operon? 20. What type of control (positive or negative) seems likely to control the production of the three enzymes? 21. What is the most likely function of the protein produced from gene R? 22. Do you expect that clc is an inducer, inhibitor, or corepressor? 23. You discover that clc binds to the protein made from gene R. What is the most likely effect of this binding? (Hint: will it cause the R protein to bind DNA or fall off of the DNA?) 24. Where do you think protein R binds? (Hint: P region or O region?)

Answer 1

For Questions 4-18, consider the mutations described in the left-hand column below. Identify whether the Lac operon will be ON or OFF in each of the environments listed across the top. The expression of the normal (wild type) operon is given. Note that "ON" and "OFF" refer to whether the operon is expressed at high levels (ON) or basal//low/no levels (OFF). (Hint: mutations render the indicated site inactive while leaving the remaining part of the operon or protein functional.) Note that question numbers are indicated within each box. No Glucose or Lactose Mutation Lactose only Glucose +Lactose Wild type operon OFF OFF ON CAP site mutated OFF OFF OFF ON ON Operator mutated ON CAP allosteric site mutated 10 OFF OFF OFF 13 14 15 Lac repressor allosteric site mutated OFF OFF OFF Lac repressor DNA binding domain mutated 17 16 18 ON ON ON

19. Yes the three genes are part of an operon.

20. Negative regulation

21. Gene R produces repressor.

22. Clc is a corepressor.

23. Clc binds to R and boost the R binds to operator
region of DNA
and inhibits the transcription process.

24. R binds to operator region O region.