assignment In order to prove that regulation by attenuation occurs in vivo, Char

Question

assignment

In order to prove that regulation by attenuation occurs in vivo, Charles Yanofsky and others
studied tryptophan synthesis regulation in a series of E. coli mutants. For each mutant
described below, predict the expression of tryptophan synthesis genes in the presence or
absence of tryptophan
Mutant A: Mutations with decreased, but detectable, tryptophan-tRNA synthetase activity
Mutant B: Mutations in AUG or Shine-Dalgarno sequence of leader peptide sequence
Mutant C: Same as mutant B but with the leader peptide fully expressed on a plasmid
Mutant D: Mutations replacing the two Trp codons in the leader peptide with Leu codons
Mutant E: Mutant D with a mutation in the Leu-tRNA synthetase gene.

Explanation / Answer

Presence of Trp Absence of Trp

Normal: no yes

Mutatnt A: no yes

The mutant will show normal phenotypes the only difference will be in amount of expression asthe levels of trp RNA synthetase has been reduced.

Mutant B: no no

Mutation in the AU or S-D sequence will prevent the expression of the operon

Mutant C: no no

The leader sequence cannot act in a trans manner, however, if the other plasmid has the whole operon then that plasmid will be able to express itself normally. But just the leader sequence in the other plasmid will not work in a trans manner.

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