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The Tryptophan Repressor
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How do repressor proteins prevent the synthesis of mRNA? Repressor proteins prevent the synthesis of mRNA by binding to DNA at a specific site near the start of a gene or sequence of genes. The bound repressor protein prevents molecules that are necessary for transcription from binding to initiation sites. RNA polymerase cannot bind to the DNA and the synthesis of RNA cannot occur. The configuration of the repressor protein must match that of the binding site on the DNA. Often another molecule must bind to the repressor to give it the proper configuration.

View the animation below, then complete the quiz to test your knowledge of the concept.





1The trp operon consists of ________ genes that encode tryptophan biosynthesis enzymes.
A)one
B)two
C)three
D)four
E)five


2In the absence of tryptophan, the trp repressor is
A)active and can bind to the operator.
B)active and cannot bind to the operator.
C)inactive and cannot bind to the promoter.
D)inactive and can bind to the operator.
E)inactive and cannot bind to the operator.


3In the presence of tryptophan, tryptophan binds to the
A)operator.
B)promoter.
C)RNA polymerase.
D)trp genes.
E)trp repressor.


4In the presence of tryptophan, transcription of the trp operon is on.
A)True
B)False


5The trp repressor is a helix-turn-helix regulator protein.
A)True
B)False





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