Phenylalanine, like tryptophan is an expensive amino acid to
synthesize. Assume regulation of phenylalanine biosynthesis is
controlled by mechanisms of gene expression similar to similar to
what we learned about for tryptophan

Draw a graph of phenylalanine biosynthesis enzyme activity
versus time for a culture of E. coli grown in a minimal
glucose medium. At t0 there is no phenylalanine in the medium. At
t2, phenylalanine is added to the medium and you continue to
measure enzyme activity to t5. (t0, t2 and t5 represent 0, 2 and 5
hours after inoculating the minimal glucose medium). Briefly
explain what is happening at each of the key time points with
respect to expression of the phenylalanine biosynthetic
enzymes.