Why does E. coli shut down the trp operon if tryptophan is available in the environment?
a. Synthesizing an amino acid takes energy so it is a waste of energy to make something that
is already available.
b. Environmental tryptophan is of higher quality than what the E. coli can make itself.
c. The trp operon encodes genes that export tryptophan from the cell; if there is already
tryptophan in the environment, further export isn't necessary.
d. It doesn't shut down the trp operon but only lowers the level of trp operon activity.
e. E. coli actually turns the trp operon on when tryptophan is present; tryptophan is an energy
source for the cell, and there is no reason to make enzymes to degrade something that is
lacking in the environment.
A
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