5. Shown below is the amino acid sequence of a wild-type protein in E. coli. Als

Question

5. Shown below is the amino acid sequence of a wild-type protein in E. coli. Also shown are the amino acid sequences of three mutant proteins that were produced by a single nucleotide change in the mRNA that encodes the wild-type protein.

Wild-type: fmet-trp-tyr-arg-gly-ser-pro-thr
Mutant 1: fmet-trp
Mutant 2: fmet-trp-his-arg-gly-ser-pro-thr
Mutant 3: fmet-cys-ile-val-val-leu-leu

A. Write one of the possible mRNA sequences for the wild-type protein.
B. Indicate the specific nucleotide change in the mRNA that led to the synthesis of each mutant protein.
C. The mRNA that produces the wild-type protein (part a) should have nine codons. What is the functional role of the ninth codon? What is the sequence of this ninth codon?
D. The mRNA molecule that produces the wild-type protein contains a 5’-untranslated region (5’-UTR) upstream of the stop codon. What sequence is contained within the 5’-UTR? What is the purpose of this sequence?

Explanation / Answer

Q.1

5' -AUG-UGG-UAU-AGA-GGG-AGU-CCC-ACC- 3'

Q.2

Mutant 1: fmet-trp

5' -AUG-UGG-UAA-3'

(UAA - Stop Codon)

Mutant 2: fmet-trp-his-arg-gly-ser-pro-thr

5' -AUG-UGG-CAU-AGA-GGG-AGU-CCC-ACC- 3'

Mutant 3: fmet-cys-ile-val-val-leu-leu

5' -AUG-UGU-AUU-GUU-GUU-CUU-CUU-UAA-3'

(UAA - Stop Codon)

Q.3

5' -AUG-UGG-UAU-AGA-GGG-AGU-CCC-ACC-UAA- 3'

UAA - Stop codon.

Translation terminates at stop codon. There are three stop codons namely UAA, UAG, UGA.

Q.4

5'-AGGAGGU-AUG-UGG-UAU-AGA-GGG-AGU-CCC-ACC- 3'

AGGAGGU - Shine Dalgarno sequence.

It is a ribosomal binding site in Escherichia coli.

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