[MUSIC] Okay, so what are the Nonsense Triplets or the Nonsense Mutants? The nonsense mutants have been identified as the ambivalent mutants, rII mutant of benzer. They have been identified as the amber mutants of HD4, and we'll talk about that next week. They also have been identified as the so called SUS mutants of phage lambda. And they have been isolated in a gene of E.coli that codes for an enzyme, alkaline phosphate. And all these mutants have in common the following property. They are mutant in one strain, and they're wild type in another strain, string dependant. What's the first thing that Brenner did was to make, what we call isogenic strings, that is string that are as closely related as possible. One which is su0 and one which is su plus, su amber and suppressor ochre and so on and so forth. And when they analyzed all the SUS, the rII ambivalent, the amber of T4 in these strains, they found that all the amber and all the SUS, and all the phosphatase mutants respond to the same suppressor. So this is a class of mutation which is independent of the gene. The phenotype, the same phenotype can be found with mutation in different genes. If you remove let's say the genes of Beadle and Tatum that we've seen in the first session. If you remove one of the genes that is important to make an amino acid like leucine, the strain does not grow without leucine, it is leu minus. That mutation is Leu minus, period. The phenotype is restricted to the leucine synthesis. [COUGH] Now these mutants are in all sorts of G including the losing G but their behavior depends on the genetic code of the organism. The set of tRNA and simple things. So there is good evidence that the nonsense mutants, the amber mutants, and then Brenner basically decided we're going to get rid of all the other nomenclature, and we're going to call them all amber which is fine with me. So what is the evidence that the amber stopped protein synthesis? There was one evidence that was very indirect. You don't make any antigen that can be recognized by antibody against the protein but that's a weak argument. So what Brenner did was to use something that he'd worked on which was the gene coding for the major protein of the bacteriophage head. A protein that is very abundant, synthesizing very high levels and you can easily detect it. Without the modern techniques, you couldn't detect it at the time. And so what he did is he looked at wild type T4 and an amber mutant. Now the amber mutant in this case is H36. And what he found is that in the H36 infected su minus or su0 cells. He gets only a piece of the protein. He get a piece that stop at this phenylalalnine. There is nothing beyond that. This is where it stops. Ala and Gly, Val Phe, Asp.Phe was a wild type, we continue with blah, blah, blah. Okay, that's already an evidence for a stop in translation. Now, he put the same mutant in a strain that is su plus, suppressor for amber. And what does he get? He gets the same peptide that stops at the phenylalanine. And a new protein that has the same size as a wild-type protein and differs by a single amino acid. This serine replaces this gluten. That's a very important result because it tells you that the stop signal is a triplet and not an extended sequence, which it could have been. Because when you suppress the stop what you get is one amino acid, which is a triplet. The triplet become red as a serine triplet. With an efficiency that is quite amazing because in this case, this is about 60%, and this is about 40%, which means that when the machine that makes proteins, the ribosome, meets the nonsense triplet. Six times out of ten it will insert a serine read it as a serine and four out of ten it will not insert anything and will stop, it's pretty efficient. So this is the biochemical part of the work. It's all done on the T4 major head protein, so in their, in the beginning of the experiment they will use a number of mutants, rII mutants. Now we are into their genetic program. And they have a lot of different suppressor that were given to them by many people in different labs. And they call from the order of isolations, su1, su2, su3, su4 suppose it got the amber, suppressors and the ochre suppressors suB, suC, suD, suE, etc. Now what do you see from such a thing? First of all, it's not clear cut, it's not black and white. There are plus 0s more or less distributed. The first thing you see is that none of the amber suppressor suppress any ochre, that's always 0. Now some of the amber suppressor is better than other, like su1 and su3 are better than the others. Fine, su3 in this set, this small set of mutants is the best because it suppresses all the amber. Now, if you look at the ochre suppressors some of them suppress most of the ochre, like suB suppress most of the ochre but not the last one, N29. This one is only suppressed here. If we didn't have a suD strain, N29 would be classified as not ochre. It's only classified as ochre because of the suD strain. Now you have to realize that this zero loss poor, which is sort of intermediate between zero and plus, what does it mean? Well, it means two things. The first thing is it means whether the amino acid, which is insurgent by the suppressor, is okay or not okay. It may be okay, it may not be okay, depending on where you are in the gene. And the second thing he says is whether the amino acid insurgent efficiency is constant or not, and it's not constant. Sometimes it's more efficient, sometimes it's less efficient. So this is what we call the context effect. So basically, you can see for instance that this in search of serine, which is [INAUDIBLE] code is S. And SuD also inserts a serine. So what I mean by context is the fact that this amber, S116 is not suppressed by su1, and is suppressed by suD in both insert the same amino acid. So that has to be an effect of context, it cannot be anything else. Okay, so they have their set of mutants a large set of amber. A large set of ochre and a large set of the present.