What is the determining factor for the transcription of structural genes of the trp operon?

This stretch of DNA is recognized by a regulatory protein known as the trp repressor. When the repressor binds to the DNA of the operator, it keeps the operon from being transcribed by physically getting in the way of RNA polymerase, the transcription enzyme. The trp repressor protein is encoded by a gene called trpR.

The answer is:

The quantity of trp repressor protein made by the regulatory gene trpR.