Genetic Regulatory Mechanisms in the Synthesis of Proteins
François Jacob and Jacques Monod.  Journal of Molecular Biology (1961) 3: 318-356

" In describing genetic mechanisms, there is a choice between being inexact and incomprehensible.  In making this presentation, I shall try to be as inexact as conscience permits"

- Professor Sven Gard in Nobel Prize presentation speech.


Author Biographies
The Operon Form of Gene Regulation
The Lactose Operon
**break**
Genetic Analysis of the lac operon
Conclusions


Author Biographies

In 1965, Jacob and Monod, shared along with André Lwoff,  the Nobel Prize in Physiology or Medicine "for their discoveries concerning genetic control of enzyme and virus synthesis”.

Francois Jacob
Born 1920 in Nancy, France.
Completed M.D. degree (1947) at Faculty of Paris, and a obtained doctorate in Science (1954) at the Sorbonne.
Joined Institute Pasteur as research assistant (1950), under Dr. André Lwoff. 
Appointed Laboratory Director (1956) and  Head of Dept. of Cell Genetics (1960).
Began collaboration with Monod (1958) at Institute Pasteur.
In 1964 became a Professor at the College de France and held chair of Cell Genetics.

Jacques Monod
Born 1910 in Paris, France. Died in 1976.
Lectured at Faculty of Science in Paris (1934) and spent time at Cal Tech (1936).
Obtained doctorate in Natural Sciences (1941) at Faculty of Paris. 
Joined Institute Pasteur as Laboratory Director (1945) with André Lwoff. 
Appointed Director of Cell Biochemistry Dept. (1954). 
Began collaboration with Jacob (1958) at Institute Pasteur.
Professor at the College de France (1967). 
Appointed Director of Institute Pasteur (1971).

Jacob and Monod proposed a series of new concepts in their paper, including, messenger RNA, regulator/operator genes, and operons.


The Operon Form of Gene Regulation

Operon: A group of genes which are transcribed together as a single mRNA.

Two types of genes found on operons:
Structural genes: Code for proteins and RNA molecules required for normal enzymatic functions in the cell.
Regulator genes: Code for proteins and RNA molecules which regulate the expression of structural genes. 
    Regulator genes are considered trans-acting since their activity can occur at a site other than where they are located  ie. at another DNA molecule
    (as opposed to cis-acting where activity must occur on the same DNA molecule)

There can also be repressors and inducers in an operon:
Repressor: A protein produced by a regulator gene which binds to a site on the operon to prevent transcription of structural genes.
Inducer: A metabolite which prevents the repressor from binding to the DNA so that the structural genes can be transcribed.
The site at which the repressor binds is known as the operator.

The Lactose Operon

Structural genes:
lacZ - codes for β-galactosidase
lacY - codes for β-galactosidase permease
lacA - codes for thiogalactoside transacetylase

Regulator genes:
lacI - codes for the lactose repressor

Also the lactose operon has an inducer - allolactose

An overview of the lac operon

Genetic Analysis of the lac operon

Jacob and Monod along with Pardee studied various mutations in order to determine how regulation of the operon works.
This experiment was called the PAJAMO experiment, named after the three scientists.

In the PAJAMO experiment:
- Two strains of E. coli were used. 
- One carried the wild type lac operon, a gene making it sensitive to streptomycin, and was a donor cell.
- The other carried the mutation of interest, a gene making it resistant to streptomycin and was an acceptor cell.
- Strains were mated and allowed to grow in medium containing streptomycin
- β-galactosidase synthesis was observed with and without inducer to determine relation of mutated gene to wild type gene.


Jacob and Monod looked at two types of mutations:
Constitutive: Mutations in which the production of β-galactosidase cannot be repressed.
Non-inducible: Mutation in which the production of β-galactosidase cannot be induced.


Looking at the mutations in order of the region of DNA they affect:

1. A 'faulty' regulator gene: I -

In this type of mutation lacI cannot produce any repressor.

Without inducer: Since no repressor is made to bind to the operator, RNA polymerase can transcribe the structural genes, β-galactosidase is synthesized.
With inducer: There is no repressor for the inducer to bind to so there is synthesis of β-galactosidase.
This kind of mutation is constitutive.

In partially diploid cells:  The functional lacI + produces repressor while lacI - does not.  This repressor works transitively to bind both operators. 
Without inducer: no β-galactosidase is synthesized. 
With inducer: it binds to the repressor and β-galactosidase is synthesized.

Relation observed:
I + is dominant to I -

How this mutation affects the lac operon


2. A dysfunctional operator: Oc

In this type of mutation the operator is changed so that the repressor cannot bind as well.  However there is still some binding by the repressor depending on the severity of the change.

Without inducer:  Some repressor can still bind in small amounts resulting in minimal synthesis of β-galactosidase.
With inducer: The inducer binds to all repressor synthesized so that β-galactosidase is synthesized in high amounts.
This kind of mutation is constitutive.

In partially diploid cells:  Repressor binds to the O+ operator, and only partially to the Oc operator. 
Without inducer: there is β-galactosidase synthesized from operon with Oc. 
With inducer: the level of synthesis increases as repressors are unable to bind either operator and both operons synthesize β-galactosidase.

Note: If the lacZ gene next to the Oc is mutated in this case, then no β-galactosidase will be synthesized since O+ is repressed and lacZ- will not produce β-galactosidase  When inducer is added, the repressor is removed from both operators and the functional lacZ will produce β-galactosidase.

Relation observed:
Oc is dominant to O+ (But only in a cis-acting form)
 
How this mutation affects the lac operon


3. A repressor which cannot bind to the operator: I-D

In this type of mutation repressor is still made from the lacI gene, but it is unable to bind to the operator.  This will not block the transcription of structural genes.

Without inducer: Since the repressor cannot bind to the operator, β-galactosidase is still synthesized.
With inducer: Whether inducer binds to repressor or not makes no difference as repressor still doesn't bind to the operator.  There is still synthesis of β-galactosidase.
This kind of mutation is constitutive.

In partially diploid cells: Functional repressor made from I + will begin to bind to the operator.  Since the repressor is transcribed as a monomer, but forms a tetramer, non-functional repressor made from I -D will begin to form tetramers with functional repressor and cause them to become non-functional. 

Relation observed:
I-D is dominant to I+

View an image of this mutation


4. A repressor which cannot bind the inducer: I S

The S stands for Super-repressor! Since this mutation is unable to bind to the inducer, it is permanently bound to the operator resulting in no synthesis of β-galactosidase.

Without inducer: Repressor is bound to the operator and transcription is blocked.
With inducer: Regardless of inducer added, repressor is bound to the operator and transcription remains blocked.
This type of mutation is non-inducible.

In partially diploid cells: Repressor made from I + and I S will bind to operators in non-induced systems.  When inducer is added, repressor from I + will leave the operator, but the I S repressor will take its place.  There will be no β-galactosidase synthesized with or without inducer.

Relation observed:
IS is dominant to I+

How this mutation affects the lac operon


Conclusions

In their original paper, Jacob and Monod had to introduce many speculative assumptions in their paper in order to come up with model for the specific control of protein synthesis.

In any case, they came up with several experimentally established conclusions from their speculations:
1.  The existence of regulator genes.
2.  Regulator gene acts via the specific cytoplasmic substance (ie. repressor) whose role is to inhibit expression of structural genes.
3.  Product of regulator gene acts directly as repressor (rather than indirectly).
4.  Chemical identification showed repressor is a RNA fraction not a protein. *
5.  Existence of operator as site of action for repressor. 
6.  Interactions of repressors with inducers or co-repressor. 
7.  Structural messenger is an unstable intermediate. *
(* We did not discuss the details of these in this presentation)

Operon theory accounted for many unexplained observations and conflicts in classic genetic theory.
Did not apply to organisms other than bacteria or even all bacteria systems. 


"The discovery of regulator and operator genes, and of repressive regulation of the activity of structural genes, reveals that the genome contains not only a series of blue-prints, but a co-ordinated program of protein synthesis and the means of controlling its execution."

- Jacob and Monod 1961
Links of Interest

Biology 2250 Lecture Notes - Dr. Steven Carr
Biochemistry 3107 Lecture Notes - Dr. Martin Mulligan
Biochemistry 4103 Lecture Notes - Dr. Martin Mulligan
Modern Genetic Analysis

Questions or Comments?  Please email Corinne Wilkerson or Jennifer Slade.