Bacteria like Escherichia coli (a friendly inhabitant of our gut) need many amino acids to survive—because, like us, they also need to build proteins.
One of amino acids they need is tryptophan.
The trp operon include 5 genes which encode enzymes needed for tryptophan biosynthesis, along with a promoter (RNA polymerase binding site) & an operator (binding site for a repressor protein).
The genes of such trp operon are transcribed as single mRNA. The Trp operon is turned on when such tryptophan levels are low.
When Trp operon is turned on, these genes are expressed, transcription is initiated and tryptophan is synthesised.
If Trp is absent inside the bacteria then amino synthesis does not occur.