[MUSIC] >> You may remember that among the strains that were used by Tatum and Lederberg, there were e coli B strains. They wanted to test whether recombination was a general phenomenon. And so they also tried B strains. And to their surprise, a B cross B strain yield no recombinant. They also tried a B cross K strains. Several K strains, also no recombinants. In fact, e coli K12 is a gift of the gods, because it provided a number of fantastic tools to study genetics. K12 E.coli B and E.coli C. Those are the four we're going to talk about. K12 carries the F plasmid or or mini chromosome. F stands for fertility. There is no F in B or in C. F is a plasmid like any other plasmid. But is a plasmid that is capable of transferring genes from one bacterium to another. This is called a plasmid capable of conjugation, or conjugative plasmid. The other conjugative plasmids, like for instance the R plasmids that carry resistance to antibiotics and were first isolated in Japan during an outbreak of antibiotic resistant diarrhea. First gift of K12. Second gift of K12, it carries a prophage, a virus in its latent form. This is a virus that was discovered by Esther Lederberg. And this is the phage called lambda. And lambda is not carried by b and c. Although all the strains carry some kind of prophage, or cryptic prophages. Prophages are element of DNA that come from a virus and become integrated in the chromosome. They are part of the plasticity of genomes. Lambdoid phages, phages like lambda, are for instance, responsible for a number of diseases. Cholera, diphtheria, are all diseases caused by plasmids carried by different bacteria. The third gift was the presence of an rpoS mutation that disfavor or inhibit survival under starvation conditions. And this mutant is in fact an amber mutant. So all of this was in K12. And the last is something called restriction. Now, e coli C carries also a restriction. But e coli C has no restriction. What is restriction? Restriction is a phenomenon by which DNA from one bacteria is destroyed in a different bacteria. If you take DNA from k12, and you inject into B, you destroy the DNA because of the restriction system. And this is a system that was analyzed and characterized biochemically by the Nobel Prize. Restriction was actually discovered in a way by Tatum and Lederberg, because you cannot exchange genetic material between B and K, B cross K. The no in B cross K is because the DNA's restricted in the recipient. This is restriction. Restriction was discovered again by Luria, when he broke his culture and used another culture, of another bacteria from a colleague, and showed that lambda would not plate on this strain because of restriction. So restriction was discovered a number of times, and by Vylet and Bertoni. >> And this is a kind of a primitive immunity against horizontal gene transfer. It's not an immunity against an antigen or against a cell, it's an immunity against the foreign DNA. The restriction of K12 is called RK and the restriction of B is called RB. So, this cannot work because of restriction. They had discovered restriction without realizing it. Now what about a B cross B strain? Why did those have the same restriction system, the same immunity? So, why can't a B give DNA to another B? Well that's because the B strains have no F, so they cannot give DNA. Now, what you have to realize is that these elements only became visible because of mutagenesis. The original K12 strain was subjected to intense mutagenesis. In some strains you lost the F while other kept the F. Some strains lost the lambda so they were became lambda sensitive. And other stayed lambda lysogen. And the same occurred here, some acquired amber suppressors. Some stayed rpos amber, some reverted to rpos plus, and some acquired an amber suppressor. All of these did not occur in this specific order, but in a way, you'd have at the end of the few early mutagenesis, you had 12 different genotypes. With F, no F. With lambda, no lambda. With rpos, with suppressor. And this was not recognized at first because people didn't know. Let Tatum and Lederberg, and Lederberg was quite lucky that his first strains, the strains that were called 58 and 769, 58 and 679, sorry. One of them was an F minus, while the other one was an F plus. If that hadn't been the case he would have not seen recombination. You can only get recombination if you cross an F minus with an F plus. So, they were smart and they knew what they were looking at, but they had to be lucky. They knew that they was a possibility that genetic recombination occurs in bacteria. What they didn't realize is the condition for that. The condition for that were realized much, much later. About six or seven years later. So this is the end of this first paper. Of recombination bacteria. Remember, some strains do recombine for unknown reasons. And the frequency of recombination is of the order of 10 to the minus 6. Low.
