Prokaryotic Gene Expression Practice Problems Solutions

1. a) Build up?
   • m1: X
   • m2: Y
   • m3: Z
   
   b) Grow on?
   
   • m1: Y, Z, or A
   • m2: Z or A
   • m3: A
   c) Build up?
   • m1,m2: X
   • m2,m3: Y
   • m1,m3: X
   (the earlier of the two)
   d) Grow on?
   • m1,m2: Z or A
   • m2,m3: A
   • m1,m3: A
   (the later of the two)

2. a) The pathway is:
   
   b) m1,m4 would grow on B or arginine.

3. The pathway is:
   

4. a) There are 3 complementation groups identifying 3 genes:
   m1, m4, and m5 are in gene 1
   m2 is in gene 2
   m6, m7, and m8 are in gene 3
   b) No. If you isolated more mutants, you might find more genes.

5. From the description of the operon, the genes must look like this:
   

   The best way to solve this type of problem is to write out the expected phenotype for the particular mutation and then match the prediction to the data. Remember that we are assuming that these are loss of function mutations (no super repressors, etc.).
   a) E1 would give LLLH. Mutation D has this phenotype.
   b) E2 would give LLHL. Mutations E and G have this phenotype.
   c) P would give LLLL. Mutations A and C have this phenotype.
   d) R or O would give HHHH. Mutations F and B have this phenotype.

   This gives the map:

   
   
   

   e) Using the gene notation from the diagram:

   Strain (1):

   Strain (2):

   Strain (3):
   

   Strains (4) and (5) deal with mutations in the operator or repressor. In this case, it is easier to predict the outcome and match it to the data:
   If in repressor:
   This looks like mutant F. Therefore, F is in repressor gene.
   If in operator: . Looks like B.

   This gives the map: