When a single promoter controls a number of genes.
If tryptophan is present in the environment, then E. coli bacteria don’t need to synthesize it, so transcription of the genes in the trp operon is switched “off.” When tryptophan availability is low, on the other hand, the operon is switched “on,” the genes are transcribed, biosynthetic enzymes are made, and more tryptophan is produced.
The trp operon includes five genes that encode enzymes needed for tryptophan biosynthesis, along with a promoter (RNA polymerase binding site) and an operator (binding site for a repressor protein). The genes of the trp operon are transcribed as a single mRNA.
The DNA is recognized by a regulatory protein known as the trp repressor. When the repressor binds to the operator at DNA it physically block the transcription enzyme.
Repressor Gene ( trp gene switch off)
The trp repressor protein is encoded by a gene called trpR. This gene is not part of the trp operon, and it’s located elsewhere on the bacterial chromosome, where it has its own promoter and other regulatory sequences.
Repressor binds to the DNA only when large amounts of tryptophan are present. there is a site for tryptophan attachement in the repressor.
When tryptophan binds to the repressor it becomes active by changing its shape.
A small molecule like trytophan, which switches a repressor into its active state, is called a corepressor.
trp Gene switch on
When there is little tryptophan in the cell, on the other hand, the trp repressor is inactive (because no tryptophan is available to bind to and activate it). It does not attach to the DNA or block transcription, and this allows the trp operon to be transcribed by RNA polymerase.
In this system, the trp repressor acts as both a sensor and a switch. It senses whether tryptophan is already present at high levels, and if so, it switches the operon to the “off” position, preventing unnecessary biosynthetic enzymes from being made.