The trp repressor blocks transcription of the trp operon (B) when the repressor binds to tryptophan.
trp operon or the tryptophan operon codes enzymes for the amino acid tryptophan. The operon is on when the tryptophan is low in amount whereas the operon is off when the tryptophan is low in amount. The operon is an assimilation of five genes.
Repressor is a small protein that is supposed to prevent the synthesis of proteins. It does so binding to the operator gene. Therefore, presence or absence of repressor is a determining factor for an operon to be on or off.
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