Prokaryotic Genetics and Gene Expression Practice Problems

1. Given the following pathway for the synthesis of the essential compound A:
   

   Mutants in genes 1, 2, or 3 (m1, m2, m3) will require A to grow on minimal medium.
   a) What intermediate will build up in the following mutants:

   • m1:
   • m2:
   • m3:
   b) What intermediate(s) will the following mutants grow on?
   • m1:
   • m2:
   • m3:
   c) What intermediate will build up in the following double mutants?
   
   • m1,m2:
   • m2,m3:
   • m1,m3:
   d) What intermediate(s) will the following double mutants grow on?
   • m1,m2:
   • m2,m3:
   • m1,m3:

2. You are studying the biosynthesis of the amino acid arginine. You have mutants in four genes (m1, m2, m3, m4) and three potential intermediates (A, B, C). You plate your mutants on plates with the following media and see if they grow (+) or not (-).

   		Medium		
   

   
   a) What is the order of enzymes and intermediates in the pathway?

   b) What supplement(s) would the double mutant m1,m4 grow on?

3. You are studying the pathway of pigment synthesis in the fictitious bacterium, Bacterium colorificus. Wild-type bacteria are red. You have three mutant strains, with altered colors.

   	Genotype	Color Phenotype
   
   	m1+ m2+ m3+	red (wild-type)
   
   	m1- m2+ m3+	orange
   
   	m1+ m2- m3+	yellow
   
   	m1+ m2+ m3-	colorless
   
   	m1- m2- m3+	orange
   
   	m1+ m2- m3-	colorless
   
   	m1- m2+ m3-	colorless
   

   Based on this information, determine the order of the enzymes and intermediates in this pathway.

4. You are studying a bacteriophage and trying to determine how many genes are required for the phage to infect and lyse the host bacterium. You have eight mutants, all of which are unable to lyse a particular host bacterium. (Note: there must be another host that can be lysed by these mutants, otherwise, you couldn't grow them!) You perform pairwise infections with each of your mutant strains and get the following result: (+) = pair of phages lysed bacterium, (-) = pair of phages failed to lyse bacterium.

   
   
   
   
   a) What are the complementation groups and how many genes have you identified?

   b) Can you be sure that you have identified all the genes involved?

5. The imaginary bacterium E. fictionalis requires two enzymes for the metabolism of the imaginary sugar froyose. Enzyme 1 converts froyose to yummose; enzyme 2 converts yummose to glucose.

   Both enzymes are synthesized from a single mRNA and are induced in response to froyose; the operon is known to be regulated by repression.

   You have isolated several mutants that are altered in their metabolism of froyose. The mutants are A through G : A indicates wild-type sequence at mutation site; A indicates mutant sequence at site A; you may assume that all mutations are loss of function that is, they inactivate the component they mutate. You have an assay for the level of enzymes 1 and 2. The results with haploid strains are shown below:

               - froyose    + froyose
   

   
   
   
   
   a) Which mutation(s) are likely to be in the coding region of enzyme 1?

   b) Which mutation(s) are likely to be in the coding region of enzyme 2?

   c) Which mutation(s) are likely to be in the promoter for enzyme 1 and 2?

   d) Which mutation(s) are likely to be in the repressor or operator?

   You then construct the following diploids:

   		 - froyose     + froyose
   

   
   
   
   e) Explain the phenotypes of strains 1 through 5. Which mutation(s) are in the repressor? the operator?

   Solutions are available.