An E. coli auxotroph A is mutated for a gene involved in synthesis of tryptophan

Question

An E. coli auxotroph A is mutated for a gene involved in synthesis of tryptophan (trp- ) and an auxotroph B is mutated for a gene involved in synthesis of arginine (arg- ). Tryptophan and arginine are both amino acids required for growth of the cell. You transform the DNA from strain B into strain A. What media would you plate the transformation experiment on to see if transformation took place?

a) Minimal media containing arginine, but not tryptophan.
b) Minimal media containing both tryptophan and arginine.
c) Minimal media without any amino acids.
d) Minimal media containing tryptophan, but not arginine.

Explanation / Answer

If the transformed cell contains both A and B genomes then the answer would be C. Normally B needs arginine and A does not need arginine. A needs tryptophan and B does not need tryptophan. So as the transformed cell has both, they supplement each other. Hence no amino acids need to be added in the medium. Non-transformed cells cannot grow in this medium. Only transformed cells can grow..

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