Biology 1961

Genetic Regulatory Mechanisms in the Synthesis of Proteins

François Jacob & Jacques Monod

Genes can switch each other on and off — a cell reads its DNA selectively.

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In depth · the introduction

Every cell in you carries the same DNA — so why is a nerve cell nothing like a skin cell? Because genes can switch each other on and off.

The big idea

Jacob and Monod studied a humble question: how does a bacterium know to make the enzyme for digesting milk sugar only when milk sugar is actually around? The answer turned out to be a kind of molecular switch built right into the DNA.

Some genes, they showed, are not blueprints for proteins at all — they are controllers. A “regulator” gene makes a small protein, the repressor, that sits down on a stretch of DNA called the operator, like a hand pressed over a switch. While it sits there, the genes next door are silenced. But if the right signal molecule shows up, it grabs the repressor and pulls it off — and the genes spring to life. The whole controlled cluster, they named the operon.

How it came about

At the Institut Pasteur in Paris, François Jacob and Jacques Monod had been chasing two seemingly unrelated puzzles — how bacteria suddenly start making a digesting enzyme, and how a dormant virus hiding in a bacterium decides to wake up. Over years of clever genetic crosses, often with their colleague Arthur Pardee, they realised both were the same story: a repressor holding a gene shut until a signal releases it.

Their 1961 review pulled it all together into one elegant model. It also made a daring side-prediction: that a gene doesn't build a protein directly but first writes out a short-lived working copy — what we now call messenger RNA — which the cell's protein factories then read. Both ideas proved foundational, and in 1965 Jacob, Monod, and André Lwoff shared the Nobel Prize.

Why it mattered

Before this, genes were imagined as a static list of instructions. Jacob and Monod showed that the list is also a program — that cells decide, moment to moment, which instructions to read. That single shift explains how one fertilised egg, with one genome, can grow into a creature of hundreds of cell types, each reading a different page. It is the conceptual root of how we understand development, and how a cancer cell is a control system gone wrong.

A way to picture it

Think of the operon as a light controlled by a motion sensor. The lamp (the structural genes) only wants to switch on when someone is in the room. The repressor is a cover taped over the switch, holding it off. The signal molecule — milk sugar, in the bacterium's case — is the person walking in: it pulls the cover away, and the light comes on. Walk out, the cover falls back, and the light goes dark. The cell spends nothing running enzymes it doesn't need.

Where it sits

This work stands on the structure of DNA (Watson and Crick, also in this Library) — it asks what the cell does with that information. It gave biology the idea of messenger RNA, the molecule that carries a gene's message and, decades later, became the basis of mRNA vaccines. And it opened the door to everything that followed: the discovery that human genes are switched by far more elaborate machinery, and to synthetic biology, where scientists now build their own genetic switches out of the very parts Jacob and Monod named.

The original document

Original source text

Abstract — a double genetic control

François Jacob & Jacques Monod · J. Mol. Biol. 3 (1961): 318–356 · received 28 December 1960

The synthesis of enzymes in bacteria follows a double genetic control. The so-called structural genes determine the molecular organization of the proteins. Other, functionally specialized, genetic determinants, called regulator and operator genes, control the rate of protein synthesis through the intermediacy of cytoplasmic components or repressors.

The repressors can be either inactivated (induction) or activated (repression) by certain specific metabolites. This system of regulation appears to operate directly at the level of the synthesis by the gene of a short-lived intermediate, or messenger, which becomes associated with the ribosomes where protein synthesis takes place.

Introduction — beyond the structural gene

1. Introduction

According to its most widely accepted modern connotation, the word “gene” designates a DNA molecule whose specific self-replicating structure can, through mechanisms unknown, become translated into the specific structure of a polypeptide chain.

It has been known for a long time, however, that the synthesis of individual proteins may be provoked or suppressed within a cell, under the influence of specific external agents, and more generally that the relative rates at which different proteins are synthesized may be profoundly altered, depending on external conditions.

[ … ]

The elective effects of agents other than the structural gene itself in promoting or suppressing the synthesis of a protein must then be described as operations which control the rate of transfer of structural information from gene to protein.

The operator and the operon

4. The Operator and the Operon

The specificity of operation of the repressor implies that it acts by forming a stereospecific combination with a constituent of the system possessing the proper (complementary) molecular configuration. … This controlling element we shall call the “operator” (Jacob & Monod, 1959).

This genetic unit of co-ordinate expression we shall call the “operon.”

It is clear that when an operator controls the expression of only a single structural cistron, the concept of the operon does not apply … It is tempting to predict that such proteins will often be found to be controlled by two (or more) adjacent and co-ordinated structural cistrons, forming an operon.

Conclusion — a co-ordinated program

6. Conclusion

The synthesis of the messenger by the structural gene is a sequential replicative process, which can be initiated only at certain points on the DNA strand, and the cytoplasmic transcription of several, linked, structural genes may depend upon a single initiating point or operator. The genes whose activity is thus co-ordinated form an operon.

The fundamental problem of chemical physiology and of embryology is to understand why tissue cells do not all express, all the time, all the potentialities inherent in their genome. The survival of the organism requires that many, and, in some tissues most, of these potentialities be unexpressed, that is to say repressed.

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.

François Jacob & Jacques Monod · Institut Pasteur, Paris · received 28 December 1960