[MUSIC] So these years were amazing. In February 1960 the operon model is on the table. At the end of the year, Jacob and Monod sent a review to Journal of Molecular Biology, and now the title is much more general, Genetic Regulatory Mechanisms in the Synthesis of Proteins. And so now, we have something that is a cathedral of modern science that integrates the work of hundreds of people who contributed to this cathedral. Among the things that are present in this, but in this very long review, is a table of analogs of lactose. This is lactose, this is phenyl-galactose and this is a galactose that has a sulfur atom instead of an oxygen, with isopropyl, methyl, phenyl, phenyllethly, various substitution and what do they measure? They measure the capacity of this sugar to induce methylal synthesis. So, lactose induces, the phenyl-lactose induces. This is the best inducer, this is IPTG. TMG that they use in the paper was not as good inducer but it is still an inducer. Now, if you look at phenyl-galactoside, it's not an inducer, this one does not induce. Phenyl or methyl TMG is not as good as isopropyl because if you go down in concentration, at 10 to the -5, you only have a small amount of induction. The point here is that some sugars are non-inducers and some are inducers. Now, what about betagal? So betagal can be measured like any enzyme by a number of different systems. We will concentrate on the V which is the speed of hydrolysis. What you see is that none of these thyl substituted sugars is a substrate. This is non-inducing, non-substrate. This is inducing, non-substrate. Of course, lactose is a substrate. And phenyl-lactose is also a substrate. So right now we have three kind of sugars. Inducer add substrate. Inducer not substrate. Not inducer not substrate. And there are some that are non inducer substrates. In fact, depending on how you use this guy, phenyl-galactoside, you can make, put less of it, so it doesn't induce and it is still a substrate. Now this tells you that the two proteins that look at lactose, must be different. Because if they were the same when you change the sugar the protein would behave the same for induction and for hydrolysis. So the pocket that binds lactose for the enzyme is like this, and the pocket the bind lactose for induction is like this. Two different sites. I didn't give you all the compound in the table because that's enough to illustrate the point. We also have in that paper a number of figures that I've not included, with a map that has a right orientation of I, Z, and Y. And they have this table. Now this table tells you the galactoside activity in use, not in use with different mutants. And they add, here, the third gene in the operon, which is transacetlyate. The order of the genes being repressor, z, y, a. But we can ignore the acetlyate, this is coordinated. This is just to show that it's coordinated. So let's go through the table. Strain number one, i+ z + y+. It's induceable for betagal, induceable for acetylate. Okay. This one is clear. If it's, i6- or i3-, these two strains are i- right? These two strains can be treated the same. Constitutive expression of both enzymes. Low inducer, weak inducer, essentially the same amount. Okay? So these are i-. They're constitutive. Now we have diploids. This f prime diploids. Stable diploids. Partial diploids. This particular one is induceable. Why? Because i3- is recessive to i+. And z can be expressed only from this chromosome. But we don't care. Likewise, this one is inducible. Because i+ is dominant over i+, and z+. This is just reversing the order of the mutation. So this is i- is recessive or i+ is dominant, whatever you want to call it. This is now a strain that has no i gene at all. So this is a constitutive strain because it has no i gene on the f, no i gene or mutant i gene on the chromosome. So, this is, this 6 is the same as number 2 and 3. Then they have a strain that is deleted on the chromosome. It's also like i-. All this is the same. This 6 and 7 do not add much. What will add is 8. What is 8? Eight is a mutant in gene i that cannot make the galactosidase. The mutation is not in the structural gene. The mutation is in the, it makes a repressor that will be incapable in reacting to lactose or IPTG to any inducer. So you've lost the site for binding lactose. And this one is what? When you make a diploid there's no enzyme. Is is dominant over i-. This was done many, many times. In a way. They had all the possible mutants. A mutant that doesn't bind DNA, I-, a mutant that doesn't bind lactose, Is. A protein that cannot bind the DNA because the operator is mutant, Oc. The only thing they didn't have here is a promoter. And the promoter, we will not have a session on the promoter. But the promoter is something which is, behaves like these all-zero mutation we've just seen and the promoter will take a long time to be discovered. And to be understood. And now the promoter is, what we call the promoter, is the place where our operon is binds and open the DNA and start to transcript. That was missed by the Jacob Monod group. The other thing that was missed by the Jacob Monod group, was the notion of the I- being recessive or not being recessive. Many, many mutants of lac I- are recessive but some are dominant. And that complicated the story because they were not prepared for that. And they did not do the right test which could have seen whether there was a real i- dominant or an oc. If you want to do the test so you need to cross the i+oc z+ with a strain, and I will do it on another slide. We will have an i- dominant o+ z+ and we will have the i+ o+ z+, the i+oc, sorry, z+. How do we distinguish them? Well you have to cross them with i+ for instance, o+ zcz. If you do this cross, you will see that the i- dominant will be dominant over the i+. You will produce constitutively, z+ and zcz, both constitutive. If you cross this i + oc z+ plus with an i+ o+ zcz, z+ will be constitutive, but cz that cz would be inducable. So you have to have when you make the diploid, you have to have a way of distinguishing both chromosomes. And the cz is what is required to distinguish both chromosomes. So we'll end with a model. And in fact, they have no way of deciding which model is correct. Whether the repressor acts at what they say the gene level or at the cytoplasmic level. Today, we're talking about acting at the gene level as being transcription. And this would be translation control. This is mRNA synthesis. This is protein synthesis. But this model is a very powerful model. It proposes that they are structural genes which we've heard before, so that's okay. Here, they moved a little bit and they call the operator a gene. Today, we call it a site. That's the genetic element that can be mutated. And regulatory genes. The regulatory gene would code for repressor. Here they propose that the repressor is an RNA because of an experiment that was poorly understood and misinterpreted. When the other repressor is a protein. And then the metabolite will either repress or induce, depending on whether it's a induceable system or a repressable system, and bind either to the operator DNA or to the operator RNA in controlled translation. There's another little mistake in this. Here they show the drawing of the messenger to be separate messengers. There's one message for gene A and one message for gene B. Today, we know that this is a single message. And in fact,t he nature of single polycistronic message will be defined using the lac system at about that time. It's not yet published. So the notion of messenger RNA which was predicted by the central dogma, was actually, at least that part, was finally settled by using of the lac system. So the lac system has been extremely helpful in In the theory making us a frame of thinking. It has been helpful for the technology because people use cloning plasmid that express bits and pieces of beta-galactosidase. It has been used in transgenic studies to look at how, where is the gene expressed in a mouse embryo, in a zebra fish embryo or a c-elegans embryo? Because you use beta-galactosidase, which you stain. So it's been used in an extraordinarily high number of systems. For the short, amusing part of the story, the cloning was a purification of single DNA molecules, an immortalization of single DNA molecules. Actually, the first not cloning, but the first purification of a gene, chemical purification of a gene, was done by using lacZ. And lacZ was the first piece of DNA that was a homogeneously purified DNA fragment. By a technique that is too complicate and we'll not enter, but it was basically a virtuoso piece, even though it wasn't really that helpful, because the revolution of genetic recombinant DNA really changed our way of doing science. So, but still, this notion of operators, sites which can bind fragments. It's a lot more complicated today, of course, but this very principle is a fundamental breakthrough proposed by Jacob and Monod in a paper that appeared barely two years after the first paper of the field, the paper that founded the field. And the big review which was going to stay for very many, large number of years because it had almost everything conceptually. Not all the technical details, but everything conceptually. That was this review that in a way is very, very closely linked to the first experimental paper. And the experimental paper has a nickname in the community because Pardee, Jacob, and Monod In that order. This paper is called the Pajamo paper, or the Pajama paper in French, and this nickname has stuck to this major contribution by these three fantastic scientists.