Question

A strain of E. coli carries a mutation that completely inactivates the enzyme encoded in the gene. Several revertants, mutants of the mutant with partly or fully restored activity, were selected, and the amino acid sequence of the enzyme was determined. The only differences found were at position 10 in the polypeptide chain.

Revertant 1 had Thr.

Revertant 2 had Glu.

Revertant 3 had Met.

Revertant 4 had Arg.

Assume that the initial mutation itself, as well as each revertant, resulted from a single nucleotide substitution.

a. What amino acid is present at position 10 in the mutant protein?

b. What codon in the mRNA would encode this amino acid?

Please provide explanations and not just correct answers.

Answer 1

As shown in question that whatever mutation happend was single nucleotide substitution which changes the whole amino acid here what we can do is we can analyse the 64 codons which translate different amino acids and can find

if we see revertant 3 which had methionine in the 64 codons one one codon codes for methionine which is AUG so we will search for most simillar and we found the following according to given information.

Threonine = ACG

Glutamate = GAG

Arginine = AGG

Methionine = AUG

Lysine = AAG

the most probable amino acid in the mutant protein present could be lysine because lysine coded by AAG in E.coli and if we do point mutation in codon of lysine we can get all other four codon for given revertants :

• AAG= lysine if second nucleotide converted to C so it will become ACG and will code for threonine as in Revertant 1.
• AAG= lysine if first nucleotide converted to G so it will become GAG and will code for Glutamate as in Revertant 2.
• AAG =lysine if second nucleotide converted to U so it will be AUG and will code for methionine as in Revertant 3.
• AAG= lysine if second nucleotide converted to G so it will become AGG and will code for arginine as in Revertant 4.