Institute of Advanced Insights Study WhatWhat HaveHave RestrictionRestriction EnzymesEnzymes EverEver DoneDone ForFor Us?Us? David T. F. Dryden Volume 10 2017 Number 4 ISSN 1756-2074 Institute of Advanced Study Insights About Insights Insights captures the ideas and work-in-progress of the Fellows of the Institute of Advanced Study at Durham University. Up to twenty distinguished and ‘fast-track’ Fellows reside at the IAS in any academic year. They are world-class scholars who come to Durham to participate in a variety of events around a core inter-disciplinary theme, which changes from year to year. Each theme inspires a new series of Insights, and these are listed in the inside back cover of each issue. These short papers take the form of thought experiments, summaries of research findings, theoretical statements, original reviews, and occasionally more fully worked treatises. Every fellow who visits the IAS is asked to write for this series. 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Where the above content is directly or indirectly reproduced in an academic context under the terms of Fair Dealing, this must be acknowledged with the appropriate bibliographical citation. The opinions stated in the Insights papers are those of their respective authors and do not necessarily reflect the opinions of the Institute of Advanced Study, Durham University, or the staff and students thereof. Institute of Advanced Study Insights WHAT HAVE RESTRICTION ENZYMES EVER DONE FOR US? Restriction enzymes, along with their counterparts the DNA methyltransferases, make up restriction-modification systems in almost all bacteria. The restriction enzymes have the ability to cut DNA into fragments. This has allowed experimenters to join such fragments together to make novel arrangements of DNA which had never before existed in nature. This ability to perform genetic engineering, developed from the 1970s onwards, has had an enormous influence on the modern world – establishing the biotechnology industry and transforming our understanding of biology and of medicine. Furthermore, the restriction enzymes have had an enormous impact on the rate of evolution of life on earth. They were almost certainly present in the earliest forms of bacterial cells some four billion years ago to control their uptake of DNA from their environment. If the restriction systems had had too much or too little control of this process then the rate of evolution of bacteria, and hence the time for the appearance of higher organisms such as ourselves, would likely have been greatly delayed – such that we might not yet exist on earth. Introduction n this paper I wish to provide two answers to the question posed by the title. The first answer Iis that modern genetic engineering, biotechnology and medicine would have been almost impossible. The second is that the restriction enzymes, ubiquitous in humble bacteria, have shaped the evolution of life on earth since the first cells arose. The bacterial phenomenon of restriction and its counterpart of modification were discovered in the early 1950s and published in two papers (Luria and Human, 1952; Bertani and Weigle, 1953). Before I explain the nature of the restriction-modification (RM) effect in bacteria, I will have to introduce the other player in this effect, namely bacteriophage (Salmond and Fineran, 2015). Bacteriophage, literally ‘bacteria eaters’, are the viruses infecting bacteria and they were first discovered by Alexander Twort (Twort, 1915) and independently by Felix d’Herelle (d’Herelle, 1917). They found that the phage, although too small to be observed with a light microscope and too small to be trapped on a filter, would, when mixed with a solution of bacteria, cause the death of the bacteria. This could be observed in a solution of bacteria, which is normally turbid, by the rapid clearing and loss of turbidity, or on a Petri dish by the appearance of holes or ‘plaques’ on a layer of growing bacteria. Phage were actually rapidly developed in the 1920s for therapeutic use to cure people and animals of bacterial infections but they fell out of use in the Western world as soon as antibiotics were discovered (they were still used in the former USSR and the Phage Institute in Georgia was particularly advanced in this area). The increasing prevalence of antibiotic-resistant bacteria is now driving a new interest in ‘phage therapy’. The scientific study of phage and their interaction with bacteria continued through the 1920s and 1930s but their physical nature and chemical composition did not become clear until the electron microscope was invented in the 1940s. Eduard Kellenberger and Jean Weigle at the University of Geneva in Switzerland showed that phage were objects of ~50nm in size with polyhedral ‘heads’, a tail and often some fibres projecting from the tail (see Bradley, 1967, for a review). To infect bacteria, they would attach themselves to the outside of the bacterial cell. The material inside the head would enter the bacterial cell and eventually more copies 2 Institute of Advanced Study Insights of the phage would be assembled inside the bacterial cell and the cell would eventually burst or ‘lyse’ to release new phage particles. The nature of the material inside the head was shown by Martha Chase and Alfred Hershey to be DNA in their famous ‘Waring blender’ experiment (Hershey and Chase, 1952). It was this DNA which was injected into the unfortunate bacterial cell. The study of phage continued for many years (Salmond and Fineran, 2015) and many tremendous advances in our understanding of molecular biology were made. However, it is now time to return to the RM effect (Luria and Human, 1952; Bertani and Weigle, 1953). The concentration of a phage solution or the number of phage particles in a defined volume such as 1 ml could be easily determined by mixing a small aliquot of the phage solution with bacteria and plating the mix on agar in a Petri dish. The number of plaques appearing would give the number of phage in the small aliquot as each plaque was the result of a single phage particle infecting a bacterium. It was noticed that the number of plaques observed on certain strains of E. coli was much less than expected. For instance, if 1000 plaques appeared on one strain (I am going to call it E. coli C), only one or two might appear on the other strain (which I will call E. coli K12) even though the same number of phage and bacteria had been put on each Petri dish. This was the restriction phenomenon and was presumed to be due to the destruction of the phage DNA by enzymes when entering a restricting strain such as E. coli K12. The plaques on these Petri dishes actually contain a large number of phage particles produced by the replication of the single original particle and these phage can be recovered by simply sticking a sterile toothpick into the plaque. Let us for the sake of argument say that 2000 phage were recovered from a plaque on the E. coli K12 Petri dish. If we mixed 1000 of them with E. coli C and plated them, then we would get 1000 plaques.