PAJAMO Experiment
Arthur Pardee, François Jacob and Jacques Monod
Experiment was key to understanding
induction of β-galactosidase.
Looked at expression of lactose operon in a partially diploid cell, in
particular, whether
I
- or I +
was dominant.
Proved Monod's "internal
inducer" hypothesis wrong.
Genes
lacZ
+
(can synthesize β-galactosidase)
lacZ
-
(cannot synthesize β-galactosidase)
lacI
+
(no internal inducer) fully inducible by external inducer
lacI -
(with internal inducer) already partly induced
Two strains of
E. Coli
Donor
strain (F+ or Hfr) is SmST6S
(sensitive
to streptomycin and bacteriophage T6).
Recipient
strain (F-) is and
SmRT6R (resistant
to streptomycin and
bacteriophage T6).
Synthesis of β-galactosidase
could only be measured in recipient strain.
Experiment
1
Z
+ I +
--> Z - I -
Expected that if
I -
is dominant, and if
I
-
provokes synthesis of
internal inducer,
then β-galactosidase
will be synthesized immediately (i.e. as soon as lacZ + gene enters
recipient).
This was observed,
BUT...
Experiment
2
Z - I -
--> Z + I + (reverse
of Exp 1)
Expected that if
no external inducer present,
then no β-galactosidase
will be synthesized until lacI -
is
transferred to the recipient and starts to make an internal
inducer.
This was not observed,
β-galactosidase
was only made if an external inducer was added.
Important
Findings
1. The lacZ
gene,
which codes for β-galactosidase,
is
expressed very fast and maximum from the beginning but it soon levels
off and stops.
Further synthesis requires addition of
external
inducer.
2. lacI
+
is dominant to lacI -
lacI -
does not code for an internal inducer.
These findings
led to the following ideas :
The
inducer does not provoke
synthesis of
the enzyme, rather it inhibits
synthesis of the "repressor"
(i.e.. negative control mechanism).
The
"repressor" is responsible for
regulating expression of β-galactosidase
and lactose permease.
The "repressor" is the product
of the lacI
gene and its function depended on the presence or absences of the
external inducer.