Question

mtr: tryptophan-specific transport protein – transports tryptophan into the cell

trpA: tryptophan synthase, a protein – synthesizes tryptophan from its basic components

cusF: Periplasmic copper binding protein – binds copper ions

tnaA: Tryptophanase – enzyme for tryptophan catabolism

trpB: Tryptophan synthase, b protein – synthesizes tryptophan

Q. If tryptophan caused the expression level to change, explain why these changes occur for each gene (that is, how does the presence of tryptophan in the media influence expression of the gene...you should be able to infer this based on each gene’s known function).

Answer 1

A tryptophan operon is a repressible operon system which regulates the synthesis of tryptophan in the bacterial cell. When tryptophan is present in the cell, it binds to the trp repressor which ultimately binds to the operator of tryptophan operon. Binding of this tryptophan repressor complex to the Operator prevents RNA polymerase from transcribing the genes within the operon. Thus, the presence of tryptophan inhibits it's synthesis by preventing the transcription of the downstream genes.

So, when the tryptophan is present in the environment as a growth media for the bacterial cell, the synthesis of tryptophan is ceased by inhibiting RNA polymerase from transcribing the genes.

In the presence of tryptophan, the bacterial cell takes it inside through transport protein called mtr​, followed by its catabolism into indole acetic acid and it's derivatives. This breakdown of tryptophan occurs via tnaA gene which encodes an enzyme tryptophanase which aids in tryptophan catabolism. Other genes like trpA and trpB are involved in the synthesis of tryptophan under the control of the tryptophan operon system.