Chapter 12, Problem 37A

Summary Introduction

To illustrate:

The effect of adding tryptophan to a culture of E. coli.

Introduction:

In prokaryotes an operon controls the transcription of genes in response to changes in the environment. An operon is a section of DNA that contains the genes for the proteins needed for a specific metabolic pathway. An operon consists of an operator, promoter, regulatory gene and the genes coding for proteins.

Answer to Problem 37A

If tryptophan is added to a culture of E. coli, the trp repressor binds to the operator and blocks the synthesis of tryptophan by preventing RNA polymerase from binding to it. E. coli will no synthesize tryptophan on its own.

Explanation of Solution

Gene regulation is the ability of an organism to control which genes are transcribed in response to the environment. The bacteria Escherichia coli (E. coli) respond to tryptophan through two operons; trp operon and lac operon.

Tryptophan synthesis is catalyzed by a set of five enzymes which are coded by five genes. The trp operon controls the transcription of these five genes. When tryptophan is present in the environment of the bacteria, the trp operon represses the transcription of the five genes because there is no need for the cell to synthesize it. An inactive repressor protein is made which combines with tryptophan and gets activated. The complex binds to the operator. If the repressor is bound to the operator, RNA polymerase cannot bind to it and this prevents transcription of enzyme genes. This prohibits synthesis of tryptophan by cell.