(9 Pts) In E. coli, three closely linked loci have been identified. The loci are symbolized by p, d
and q. Wild type alleles are superscripted with a plus, while mutant alleles are superscripted with
a minus. When an inducer is present or absent, the level of Enzyme X is measured and the results
for several different strains are given below From the information given, determine which of the
genes is the regulator gene, which is the operator and which is the structural gene for Enzyme X.
Note: There are no super-repressor mutants and assume all promoters are wild type! Phenotype
(level of Enzyme X) Absence of Inducer Presence of Inducer Genotype p+d-q+ p- d+qt p+ d+q-
p+ d+ q+/p- d-q- p+d-9+/p-d+q none none high high none high high high none high none high lg
ig high high high lg p-d+q+/p+ d-q-
Solution
When d is absent, no enzyme is made. So, d can not be the regulator gene (repressor). It can be
the structural gene.
When p is absent, the enzyme is synthesized irrespective of the presence or absence of inducer.
So, p must be the regulator gene that can sense and respond to the inducer. Since p is a trans-
acting factor, it can be complemented.
When q is absent, the enzyme is synthesized irrespective of the presence or absence of inducer.
So, q must be the operator sequence that can be bound by the repressor. Since d is a cis-acting
factor, it can not be complemented.
d = Structural gene
p = Regulatory gene
q = Operator sequence
and q. Wild type alleles are superscripted with a plus, while mutant alleles are superscripted with
a minus. When an inducer is present or absent, the level of Enzyme X is measured and the results
for several different strains are given below From the information given, determine which of the
genes is the regulator gene, which is the operator and which is the structural gene for Enzyme X.
Note: There are no super-repressor mutants and assume all promoters are wild type! Phenotype
(level of Enzyme X) Absence of Inducer Presence of Inducer Genotype p+d-q+ p- d+qt p+ d+q-
p+ d+ q+/p- d-q- p+d-9+/p-d+q none none high high none high high high none high none high lg
ig high high high lg p-d+q+/p+ d-q-
Solution
When d is absent, no enzyme is made. So, d can not be the regulator gene (repressor). It can be
the structural gene.
When p is absent, the enzyme is synthesized irrespective of the presence or absence of inducer.
So, p must be the regulator gene that can sense and respond to the inducer. Since p is a trans-
acting factor, it can be complemented.
When q is absent, the enzyme is synthesized irrespective of the presence or absence of inducer.
So, q must be the operator sequence that can be bound by the repressor. Since d is a cis-acting
factor, it can not be complemented.
d = Structural gene
p = Regulatory gene
q = Operator sequence
