Chapter 8, Problem 51P

You are studying mutations in a bacterial gene that codes for an enzyme whose amino acid sequence is known. In the wild-type protein, proline is the fifth amino acid from the amino terminal end. In one of your mutants with nonfunctional enzyme, you find a serine at position number 5. You subject this mutant to further mutagenesis and recover three different strains. Strain A has a proline at position number 5 and acts just like a wild-type strain. Strain B has tryptophan at position number 5 and also acts like wild type. Strain C has no detectable enzyme function at any temperature, and you can’t recover any protein that resembles the enzyme. You mutagenize strain C and recover a strain (C-1) that has enzyme function. The second mutation in C-1 that is responsible for the recovery of enzyme function does not map at the enzyme locus.

a	What is the nucleotide sequence in both strands of the wild-type gene at this location?
b	Why does strain B have a wild-type phenotype? Why does the original mutant with serine at position 5 lack function?
c	What is the nature of the mutation in strain C?
d	What is the second mutation that arose in C-1?