Question

The regulation of tryptophan synthesis in E. coli. is an example of affecting gene expression through:
A) Translational control
B) Transcriptional control
C) Homeotic gene control
D) Breaking down mRNA molecules

Answer 1

Hint: The presence or absence of tryptophan in the environment largely affects the tryptophan synthesis in E. coli. If the tryptophan concentration is high in the environment, then the bacteria will not need to prepare using its machinery.

Step by step solution:
Tryptophan is an essential amino acid which is required to produce essential proteins. In E. coli , tryptophan is produced by 5 genes which are placed next to each other. This is called as trp operon
If the tryptophan concentration is high in the environment, then tryptophan binds to a trp repressor protein. Trp repressor binds to gene and stops or represses its expression. After binding with tryptophan, the repressor changes its structure and makes it active. This active molecule can then bind to the DNA to repress the expression.
If tryptophan concentration is low, then the repressor becomes inactive and does not bind to the genes in Tap operon. This leads to their expression and tryptophan proteins are produced.
Transcription is the process where a gene is transcribed to mRNA. mRNA is then translated to form proteins. In this case, the tryptophan concentration affects the repressor binding to DNA. This affects the transcription of the gene. Hence, regulation of the tryptophan synthesis is controlled by transcription.

So, option B is correct.

Note: Operon contains a set of genes which are controlled by a single promoter. They also contain sequences and sites for the binding of regulatory proteins such as repressors. This transcriptional control helps in energy minimization in the cell.