[MUSIC] Today we're going to discuss another kind of bacterial viruses. The lysogenic viruses. So far, when spoke about T4, T1, all of these were lydic viruses. That is, they infect, and 20, 30, 40 minutes later, the cell bursts and liberates 40, 50, 100, phage particles. Lyzogeny is a very different system because with lyzogenic phages, the phage can choose between two developmental programs. Lyzes, lytic cycle versus lyzogen. This notion of lisogenic viruses was discovered extremely early, actually in the beginning of the 1920's. But people had a lot of difficulties in studying the phenomena and lysogenesis was considered to be some kind of an obscure phenomenon for many years. In particular, Max Delbrück was adamant that people should not work on lysogenic phages, because it was too complicated. Burnet who worked on lysogenic viruses as well as on lytic viruses in the '30s used a word from embryology, he called the state of the virus when it is carried, not actively propagating, he called it an analogue. He used a term from embryology. One of the first people who actually had some hints about the genetic implication of lysogeny was the father of an evil man who we discussed a couple of weeks ago. So, this was a very murky field until 1949 when Esther Lederberg the wife of Josh Lederberg discovered Fage Lambda. How did she discover Fage Lambda? Well, you may remember that we had all this different strains derived from E coli K 12, some had the F factor, the fertility factor and some of them had lost the F factor. And they were called f minors. So, we have the f plus and the f minus cells. Okay, using the same strains, Esther Lederberg observed that if you take, you try to plate cells, and you mix cells from two different strains, ff course you can get recombinance. That's what Josh Lederburg and Tatum did. But you can also get plaques. Plaques derived from phage. And she didn't exactly know what to do. She called this phage lambda. Because it look a little bit like a genetic element discovered in paramecia called kappa. Lambda was the next letter. At about the same time, Bertani, who was a young post-doc at Cold Spring Harbor observed that some of his colonies, he was plating bacteria, some of his colonies were. They were sort of eaten. Slightly eaten. And they were no longer round, perfect colonies. So he'd never seen that. So he went to his neighbors and his neighbors were Evelyn Witkin who was a great lady working on UV mithogensis and UV repair. And the other colleague was Gus Durman and Gus Durman was actually the founder of the Rochester School of Genetics from which the capstein, Frank Stall and many other, Edgar and all these people came. So they said, well you probably, your bacteria are contaminated and then one of them, and Bertoni does not remember who, said well maybe this has got something to do with lisoginy. And so Bertani went to look in the library and read a few paper's and he became interested in this phenomenon because it was an obscure phenomenon. And so in 1950 he moved to the lab of Luria and decided that he wanted to work on lysogen. So Luria was not exactly very keen, but well he let him start and he had heard about the Lederberg result, the lambda phage and so they got the lambda phage and lambda sensitive bacteria from leather work. You have to realize that if you want to study a phage that is isogenic, you must have a strain of bacteria that is susceptible to this phage but is not lysogenic. Otherwise you will never see the plaques. If you plate E coli K12, the original E coli K12, you don't see the plaques because the phage will not propagate in a lysogen. This is called immunity. So that was the beginning of Bertanis's work. And then came a big, big event for the study of lysogeny. Oliver Buff who was the head of the microbial physiology unit at the Pasteur managed to do a very important experiment. Not with E coli, but with another bacterium called bacillus megaterium, which has an advantage of being very large, much larger than E coli. So using this bacterium both devised a technique to follow individual donor cells from that particular bacterium in micrograms under the microscope with tiny forceps. So, he would use the forceps to move one of the daughter to a new drop, keep the first daughter. When the two daughters would divide, he would move the two daughters in two. And so, he'd have four drops, etc. When he did that, he discovered that sometime, and actually quite frequently, one of the bacteria disappeared, From under his eyes. It lysed. And in the drop he had another strain of bacillus which was not lysogenic. In the drop he could find the phage. So this experiment the abrupt, spontaneous occasion and spread of virus was very, very important. Because it was not like some kind of continuous secretion of viral particles from the bacteria. This was a burst like what happens with. Very quickly after that, and a few others observed that which was a relatively rare phenomenon could be induced by agents such as UV. So when you treated a culture with UV, all the cells would lysis. It was a synchronized lysis. These two papers by Wolf and his colleagues, were extremely important for the study of lysogenic phages because they meant that these phages became real objects of study. And so at this time, Bertani was in Luria's group, and he had the choice of working on lambda, which the Luria lab obtained from Lederberg, or on working on another system that was also obtained from Lederberg. Now the first system was Lambda. And although Lederburg and and her husband had given all the strains and the phage through the Luria Lab, they were quite adamant that this was their subject. They wanted to work on it and they were not very keen on having somebody else working on it as a competitor. So, Luria, since the relationship between all these people is really good, Luria thought they was would be not fair to let that time to continue. So Burtani shift to the other system. Now, the other system was a system from E coli strain called Lisbon. Lisbon in [FOREIGN] was E coli strain, isolated in the '20s, worked in the pasture and for which there was a good presumption that phage's existed. The problem with the Lisman strain was the indicator. The indicator strain for the Lisman phage, was and is a shigella disenterior bacteria. Now this is a bacterium that's called Shigella which is a pretty severe gastrointestinal disorder, you can actually die from it, and so you have to be a bit careful. So there was a lab meeting and Bertani had just arrived a few weeks ago. And there was a a PhD student in the Luria Lab, who was in his last year. Was finishing, and was going to leave anyway. And this person was completely furious at the idea that people would work with Shigella. And this guy was Jim Watson. And he declared at the top of his voice that he would not want to be in a lab where one used routinely the pathogenic shigalla. So they had to ensure some extra precaution. In fact you can work with shigalla as long as you don't drink it. You don't really risk anything. So, Bertani started to work on this E coli Lisbon strain using the Shigella indicator. And that's how he discovered the phage we're going to discuss, after you read the paper. The phage are called P1, P2, and P3. We're going to concentrate on P1. And P1 became a very popular phage, mostly thanks to a gentleman named Lennox, who was a physicist, who decided to change into biology, and went to work with Bertani. And he had read some things in the library. And he'd found that Zinder and Lederberg had discovered transduction, transfer of genetic information from one material into another via a phage. And so he told Bertani, why don't we check whether P1 is a transducing phage? They had absolutely no reason to do the experiment. Only somebody who just started, could be so fresh, and young, and open. And in fact, p1 is one of the best transducing phage. It's used continuously today to construct strains. And it also gave the community a fantastic tool, which is called the Cre-lox system, which he used to selectively and at will remove a gene from mice in a specific cell type or at a specific time. So the Cre-lox to the combination system is extremely popular, extremely used and it's all based on the work of P1.