Question

In E. coli, three structural genes, A, D, and E, encode enzymes A, D, and E, respectively. Gene O is an operator. The genes are in the order O‑A‑D‑E on the chromosome and might form an operon. These enzymes catalyze the biosynthesis of valine. Mutations were isolated at the A, D, E, and O genes to study the production of enzymes A, D, and E when cellular levels of valine were low (T. Ramakrishnan and E. A. Adelberg, 1965). Levels of the enzymes produced by partial‑diploid E. coli with various combinations of mutations are shown in the table. The wild type is indicated with a (+), and the mutant is indicated with a (−).

What type of regulator protein is binding to the operator in this possible operon?
a repressor
a coactivator
an attenuator
an inducer
an activator
Are genes A, D, and E all under control of the operator O?
a.Yes, because when the operator is nonfunctional the genes E, D, and A expression levels are high.
b. Yes, because when the operator is functional the genes E, D, and A expression levels are high.
c. No, because genotype 4 shows that gene A is controlled separately from genes E and D.
d. No, because genotype 5 shows that gene E is controlled separately from genes A and D

Answer 1

A repressor protein binds to the operator in the possible operon, inhibiting transcription of structural genes A, D, and E unless an inducer is present. The expression levels of these enzymes are high when the operator is nonfunctional, indicating that all three genes are typically regulated by the operator O.

Step-by-step explanation:

The type of regulator protein that is binding to the operator in this possible operon is a repressor. In prokaryotic gene regulation, such as in the E. coli lac operon model, a repressor protein can bind to the operator sequence to prevent transcription when certain cellular conditions are met, usually in the absence of a specific inducer molecule. In the presence of an inducer, the repressor is unable to bind to the operator, allowing RNA polymerase to initiate transcription of the structural genes adjacent to the operator.

As for whether genes A, D, and E are all under the control of the operator O, the correct answer would be (a) Yes, because when the operator is nonfunctional the genes E, D, and A expression levels are high. This pattern suggests that the normal function of the operator is to hold back or repress gene expression when the repressor is bound, and when it is nonfunctional due to a mutation, the repression is lifted, allowing for higher expression levels of the associated structural genes.