The *E.coli* trp operon codes for five genes required for tryptophan synthesis. When tryptophan in the cell is low, the trp operon is expressed. When tryptophan levels are sufficient, the operon is repressed. The trp repressor binds directly with tryptophan which causes a conformational change in the repressor that allows it to bind the operator sequence and repress transcription.

trp Operon

Repressible operons typically code for proteins needed to make products required for cellular pathways. When the cell requires the product of this operon, this operon will be expressed. But once the product is accumulated in the cell, it binds the repressor of the operon and the operon is repressed or turned off. This regulates the amount of products in the cell through gene regulation and expression. 

University of Wisconsin - Madison

Operons are a section of genes that are all transcribed together. The operon is regulated by the promoter and operator sequence upstream of the genes in the operon. Operons are found primarily in prokaryotes.

Operon

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Repressible Operons

Inducible operons typically encode proteins that use or break down specific molecules in the cell. Their expression is induced when in the presence of that molecule so that the operon is only expressed when the genes are needed to break down that compound.

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