Leading molecular biologists from around the world came together at a conference jointly organized by the European Molecular Biology Organisation (EMBO) and the Institut Pasteur in Paris between 17th and 20th May to celebrate the 50th anniversary of the operon concept published by François Jacob and Jacques Monod in 1961. The scientists discussed concepts and research results influenced by the operon model, which have led to our present understanding of the highly complex interactions between the regulation of gene function and phenotypic appearance.
In June 1961, François Jacob and Jacques Monod published an article entitled “Genetic Regulatory Mechanisms in the Synthesis of Proteins” in the Journal of Molecular Biology. In this publication, the authors presented a model for the regulation of gene expression, which later became known as operon model, and which had a decisive impact on the then emerging field of molecular biology. To celebrate the 50th anniversary of the landmark publication, the European Molecular Biology Organisation (EMBO) and the Institut Pasteur jointly organized a conference entitled “The Operon Model and its impact on Modern Molecular Biology”. The list of speakers who reviewed our present knowledge on gene expression control, etc. at the symposium held in Paris between 17th and 20th May 2011, reads like a “Who’s who” of cell and molecular biology of the last decades. Even the venerable Institut Pasteur has rarely been host to so many distinguished scientists, among whom were the Nobel Laureates Elizabeth Blackburn, François Jacob, Paul Nurse, Christiane Nüsslein-Volhard, Philip Sharp and Ada Yonath.
The operon model put forward for the lacZ gene that codes for the E. coli enzyme β galactosidase was a conceptual breakthrough by Jacob and Monod, as it formulated for the first time the hypothesis that genes are regulated at the transcriptional level by specific regulatory genes and their products (RNA or proteins), and that these regulators respond to metabolic changes, environment, etc. It also emphasized the role of a newly discovered class of RNA (messenger RNA) as an intermediate between genes and their protein products. It is therefore no accident that the EMBO and the European Molecular Biology Laboratory (EMBL) conference centre in Heidelberg was named “Operon”. The conference programme, which was put together by a scientific committee that included Margaret Buckingham of the Institut Pasteur, Lucy Shapiro of Stanford University and Hermann Bujard (see BIOPRO article “Hermann Bujard - a passionate basic researcher” of 12th February 2008), former EMBO director and founding director of the Centre for Molecular Biology in Heidelberg, covered a broad range of topics, including historical perspectives of the mechanisms of transcription, epigenetics, cell differentiation, growth and division as well as the latest concepts of gene expression regulation through microRNAs and genetic networks. The new director of EMBO, Maria Leptin from Heidelberg (see BIOPRO article “Maria Leptin – the first woman scientist at the head of the renowned EMBO” of 14th November 2010) introduced the topic to the conference participants, many of whom are or have been associated with the EMBO or EMBL, and, Professor Bujard closed the conference by summarising the many developments that have arisen from the operon model.A lecture by François Jacob, who is already well into his 90s, was the emotional highlight of the conference. Jacob reviewed the decisive experiments that led him and Monod to develop their operon model, for which they were awarded the Nobel Prize in Physiology and Medicine in 1965. Monod died in 1976 from leukaemia, but one of his scholars, Jean-Pierre Changeux, who was also decisively involved in the development of the operon model, was at the conference.
The operon paradigm had groundbreaking effects on our conception of genes, suggesting that, in addition to “structural genes”, i.e. the carriers of structural information for the production of proteins, there are also “regulatory genes” that are able to activate or inhibit the expression of structural information. A third element of DNA is a signal sequence (binding site that is not transcribed) that recognizes the metabolic signal that leads to the regulation of genes and gene products (some time later, promoter sequences were discovered; these sequences are the targets of the RNA polymerase enzyme).While Jacob and Monod’s model was still close to the concept of the gene as it was perceived in classical neodarwinistic genetics according to which it leads to a specific phenotype and this phenotypic trait represents a target for evolutionary selection, it nevertheless marked an expansion of the neodarwinistic belief towards the further development of the notion of DNA as a “programme”, thus questioning the classical concept of “the gene”. Jacob himself contended that the three elements – structural genes, regulatory genes and signal sequences – provided the framework for viewing the phenotype as an ordered hierarchical system (genetic programme) that needed its own products for being executed: “Only the incessant execution of a programme is inseparable from its realization. For the only elements that are able to interpret the genetic message are the products of that message” (Jacob, François. 1976. The Logic of Life. New York: Vanguard).
When scientists around the world eventually started investigating organisms that were more complex than bacteria, they realized that the relationship between genes and their phenotypic expression was far more complicated than previously assumed. Since the beginning of the 1960s, the picture of gene expression has become a lot more complicated: many other genes and elements have been discovered that allow a cell to express a protein when needed. The conference neatly showed the many impacts of such regulatory genes or elements on gene expression and genomic complexity: The “jumping genes” and transposable elements discovered by Barbara McClintock are sequences of DNA that can move or transpose themselves to new positions within a cell’s genome. Genes can also code for multiple proteins. This is because eukaryotic genes consist of exons and introns; when a gene is transcribed, the exons are reconnected in multiple ways in a process known as alternative splicing, which increases the diversity of proteins that can be encoded by the genome. The rearrangement of genetic material and different modular combinations in somatic cells enable vertebrates to produce a plethora of different antibodies; no single genome would be big enough to harbour the information required for the generation of this diversity. The duplication of genes can lead to huge gene families in which individual genes are used for completely different functions; gene silencing, i.e. the switching off of a gene, might lead to pseudogenes that have lost their protein-coding ability or are no longer expressed in the cell. Other genes differ only slightly from each other and can produce different protein isoforms. Prior to the molecular biology era, the diversity and evolutionary selection of genes was mainly assumed to be due to point mutations; it is now known that there is a huge range of mechanisms that enable “genetic tinkering”, as Jacob called it, at the genomic level.
Developmental genes such those of the Hox and Pax master control genes investigated by Prof. Nüsslein-Volhardt and other researchers can be compared to the regulatory genes of the operon model. The Hox genes determine the basic structure of an organism and the Pax proteins are important in early animal development for the specification of tissues and organs. However, there is one important difference between the Hox and Pax genes and the operon model, as Müller-Willie and Rheinberger have suggested: while the regulatory genes are able to reversibly switch on and off the structural genes that they control, the developmental genes trigger irreversible differentiation processes. Regulatory mechanisms such epigenetics, which were discussed in depth at the conference, have further increased the old notion about the inheritance of genes. Epigenetic gene expression patterns (e.g., the histone acetylation and DNA methylation) can stay the same through cell divisions for the remainder of a cell’s life and may also last for multiple generations in sexually reproducing organisms. Such epigenetic changes are caused by mechanisms other than changes in the underlying DNA sequence and enable, at least to a limited degree, the inheritance of acquired characteristics, which recalls Lamarck’s evolutionary theory that physiological changes acquired over the life of an organism may be transmitted to offspring.
The discovery of the enzyme reverse transcriptase has shown that the “Central Dogma of Molecular Biology” articulated by Francis Crick (genetic information is transmitted from DNA to RNA and from there to protein, but never in the opposite direction) is not absolutely valid. Some researchers now believe that the synthesis of functional three-dimensional proteins from one-dimensional gene sequences is only possible when cells have also inherited the entire highly complex translation machinery (ribosomes, t-RNAs and hundreds of enzymes) and the chaperones. These researchers claim that functional three-dimensional proteins not only arise on the basis of DNA and mRNA, but also require the entire molecular environment. Susan Oyama’s developmental systems theory shifts attention away from genes and the environment and attaches greater importance to developmental systems. The logic of this model might satisfy some philosophers, but it does not satisfy molecular biologists who are well aware that genetic engineering has achieved huge application and commercial success with the concepts of individual and functional genes. There are very few experimental approaches available to test the developmental systems theory. However, a closer analysis of the successes achieved by genetic engineering shows that they only reflect the function of genes at first sight. In the majority of cases, the phenotypic expression of genes depends on the complex interactions and networks of genetic programmes such as those postulated for the first time by Jacob and Monod for a simple bacterial system around half a century ago.
Literature:Staffan Müller-Wille and Hans-Jörg Rheinberger: Das Gen im Zeitalter der Postgenomik. Edition Unseld 25, Suhrkamp, Frankfurt am Main, 2009