Problem 1. For E. coli strains below, use (Y) or (N) to indicate whether a functional protein is being produced by b-galactosidase and permease genes for each of the environmental conditions. Assume that the second copy of lac operon (where indicated) is carried on an F' factor.
|
|
Lactose absent
|
Lactose present
|
|
Strain genotype at lac
|
β-galactosidase
|
permease
|
β-galactosidase
|
permease
|
|
I+ P+ O+ Z+ Y+
|
|
|
|
|
|
I- P+ O+ Z+ Y+
|
|
|
|
|
|
I- P+ O+ Z+ Y-
|
|
|
|
|
|
I- P+ OC Z+ Y+ / I+ P+ O+ Z- Y-
|
|
|
|
|
|
I- P+ O+ Z+ Y- / I+ P- O+ Z- Y+
|
|
|
|
|
|
I+ P- OC Z- Y+ / I- P+ O+ Z+ Y-
|
|
|
|
|
|
I+ P+ O+ Z- Y+ / I- P+ O+ Z+ Y-
|
|
|
|
|
|
IS P+ O+ Z+ Y- / I+ P+ O+ Z- Y+
|
|
|
|
|
|
IS P- O+ Z+ Y- / I+ P+ OC Z- Y+
|
|
|
|
|
Q1. In a mutant E. coli strain, CAP site of lac operon is mutated so the CAP protein can no longer bind. Will this mutation slow down culture growth in glucose-supplemented media?
Q2. Will trp operon be responsive to tryptophan levels if trpL is deleted?
Q3. Why attenuation can occur in bacteria but not in a eukaryotic cell?
Q4. Under what conditions would trpR- mutant E. coli strain synthesize tryptophan?
a) constitutively
b) never
c) when tryptophan levels get so low that ribosomes pause at UGG codons
Q5. Attenuation inhibits trp operon expression in conditions of sufficient tryptophan levels via:
a) terminating transcription
b) terminating translation
c) blocking co-repressor binding to trpR-encoded repressor
Q6. Will attenuation occur under the following conditions? Give one-sentence rationale to your answer. Assume that these mutations disrupt base-pairing with partner regions.
a) Region 2 mutated, tryptophan is low
b) Region 3 mutated, tryptophan is high
c) Uracil-rich region acquired several guanines, tryptophan level is high
Problem 2. Certain mutations of lacI repressor gene cause the loss of its function, while others create a "super-repressor" protein. Propose a hypothesis regarding what parts of the protein can be altered to produce these two opposite mutant functionalities. Provide a brief rationale for each hypothesis