In E. coli , three structural genes ( A , D , and E ) encode enzymes A, D, and E
ID: 3166733 • Letter: I
Question
In E. coli, three structural genes (A, D, and E) encode enzymes A, D, and E, respectively. Gene Ois an operator. These enzymes catalyze the biosynthesis of valine. Mutations were isolated at the A, D, E,and Ogenes to study the production of enzymes A, D, and E when cellular levels of valine were low.Levels of the enzymes produced by partial-diploid E. coliwith various combinations of mutations are shown in the following table.
Genotype
Amount of enzyme E produced
Amount of enzyme D produced
Amount of enzyme A produced
E+D+A+O+/E+D+A+O+
2.40
2.00
3.50
E+D+A+O-/E+D+A+O+
35.80
38.60
46.80
E+D-A+O-/E+D+A-O+
1.80
1.00
47.00
E+D+A-O-/E+D+A+O+
35.30
38.00
1.70
E-D+A+O-/E+D-A+O+
2.38
38.00
48.70
What type of regulator protein in this operon is binding to the operator?
A repressor
An activator
A coactivator
An inducer
An insulator
*Please explain the concept behind the answer, it was not explained well in class and there are no complex examples in the book. Thanks in advance*
Genotype
Amount of enzyme E produced
Amount of enzyme D produced
Amount of enzyme A produced
E+D+A+O+/E+D+A+O+
2.40
2.00
3.50
E+D+A+O-/E+D+A+O+
35.80
38.60
46.80
E+D-A+O-/E+D+A-O+
1.80
1.00
47.00
E+D+A-O-/E+D+A+O+
35.30
38.00
1.70
E-D+A+O-/E+D-A+O+
2.38
38.00
48.70
Explanation / Answer
The E. coli has 3 structural genes encoding enzymes A, D and E. These genes are expressed as an operon and genes are under control of operator. A mutation in all 3 genes and operator gene O is also identified. Basing on the enzyme concentration when the cellular valine levels are low, it can be concluded that the genes are transcribed producing high amounts of enzyme. This operon is regulated by regulatory protein called repressor. in normal conditions, the repressor binds to operator and turns off the transcription of genes in presence or absence of valine.
Here, in this condition, the operator gene O has a mutation and though the repressor bound to operator , it could not bind with correct conformation and therefore, no efect on transcription. This means, repressor binding to operator cannot turn off transcription. The mutation of the operator gene o allows transcription of these three genes A, D and E though the operator O is bound with repressor. Therefore, option 1 is correct.