Jacob, Monod, the Lac Operon, and the PaJaMa Experiment-Gene Expression Circuitry Changing the Face of Cancer Research.

See related article by Pitot and Heidelberger, Cancer Res 1963;23: 1694–700. It is a virtually universal rule in science that if we step back to reflect upon a field currently viewed as extremely dynamic and novel, we find ourselves standing on the shoulders of thosewhose seminal observations gave birth to it far earlier. For those of us working in the fields of signal transduction and epigenetics within the cancer research arena, this is absolutely the case when we consider the brilliant realizations of Jacob and Monod that regulatory networks control gene expression in bacteria (1–5). Their recognition that expression of a single gene can be repressed by another gene for response to regulatory cues from the environment ranks as one of the top, and nowmost heavily explored, areas of biology in general and cancer biology. A review in Cancer Research in 1961byPitot andHeidelberger notonly pays tribute to this pivotal work of Jacob and Monod but with the prescient intent of predicting how the concepts might be woven into our understanding of carcinogenesis (6). To say their predictions were accurate would be an understatement, as is readily apparent from today's marriage between the exploration of regulation of gene expression and our current efforts to dissect basic mechanisms underlying the origins, initiation, and progression of cancer. It becomes evident as well that the studies of Jacob and Monod, and their implications as visualized by Pitot and Heidelberger, helped usher in a biology that underpins our current quest to evolve new strategies for improving the management of cancer. Hence the selection of the review by Pitot and Heidelberger for inclusion in the current celebration of 75 years of publishing in Cancer Research. The revelations provided by Jacob and Monod started, as do many great stories in science, with a series of epiphanies by the younger investigator, Jacob, which he brought to conversations with the more established scientist, Monod. They followed their eventual joint excitement over the possibilities raised with a series of experiments, conducted during 1958 through 1961 at the Pasteur Institute in Paris. These resulted in their outlining a model for gene regulation, which survives as a core paradigm today. Their observations established the principle that to properly regulate response of an organism to changing environmental conditions, in specific bacteria for their experiments, a gene circuitry exists wherein one gene product regulates control of another gene. The result is a change in cellular phenotype for cellular metabolism (2–5). Building on experiments for demonstrating that lambda phage genes can be both induced and repressed in bacteria, the investigators established that changes in need for lactose utilization lead to negative regulation of b-galactosidase (2–5). The circuitry for this switch formed what is now famously knownas the lac operon (1–5). The studies took advantage of the mating system employed in bacteria, in which the chromosomal material of the male is progressively injected over time into the female, thus progressively carrying genetic material with it. This allowed investigators to map male genes by chromosome position as their entry facilitated gene expression events in the female. Toning down the sexual connotations for the literature, the seminal study of Jacob and Monod, with participation of Arthur Pardee, was first published as a preliminary report in 1958 where it was dubbed the "PaJaMa" experiment (1, 3, 5). In this study, the investigators were able to show that a gene lacl encoded a trans-acting repressor for the lac gene. In this concept, the activity of the regulator gene is induced when the repressor protein in the cytoplasm is induced by a small molecular weight product generated by the target enzyme. This circuitry paradigm contributes robustly to mechanisms for pathway feedback inhibition. The nature of the trans-acting molecules through which the repressive process is mediated remained to be determined with many discussions of whether direct DNA–DNA interactions, RNA, proteins, etc., would play this role. These subsequent discussions, held at the headiest of meetings attended by many luminaries in the embryonic field of molecular biology, are credited with leading to the discovery of mRNA as put forth in a review by Alexander Gann (3). Herein is described a lunch in Sydney Brenner's rooms in King's College on Good Friday, now some 55 years ago, attended by Jacob, Brenner, Francis Crick, Alan Garen, and others where "suddenly that afternoon it became obvious—first to Brenner and Crick, and then to the others present—that the PaJaMa experiment predicted an unstable intermediate in gene expression," which was concluded to be RNA. This suggested to the attendees that the mediator for the repressor action potentially "really did act at the genetic level controlling production of the unstable mRNA. This discussion, continued that evening at a party at Crick's house, led directly to the experiment by Brenner and Jacob, who, together with Matt Meselson at Caltech that summer, demonstrated the existence of mRNA. Separately, Jim Watson, Wally Gilbert, and Francois Gros arrived at a similar result through different means at Harvard" (3). The years to come in our current age of biology have revealed that all of the hypotheses derived from the first findings of Jacob and Monod were relevant and presaged the findings of how many different ways such transacting events can be molecularly mediated. In decades following the above observations, the paradigm of the lac operon and its constituent repressor binding to an operator and inducer ushered in an era, ever growing today, for our understanding of cellular control through signal The Sidney Kimmel Comprehensive Cancer Center at Johns Hopkins, Baltimore, Maryland.