The natural ribozyme I chose to work with, the so-called 'hammerhead ribozyme' which is found in certain plant viruses which use them in rounds of replication and there were one or two other people working on it, so Bill Scott said he would work on that and in the end he... we prepared the hammerhead enzyme which was about 70... it's about the same size as the tRNA actually, and he crystallised it and out of the... follow this chain. Now there had been work in parallel by a man... [Dave] McKay at Stanford, now in order to stop the ribozyme acting, what he had done was to replace one of the chains of the hammerhead sequence by DNA rather than RNA. Now we tried to stop the action by changing the pH of our hammerhead RNA but in fact it was... they did do that but you couldn't get to such good crystals when we changed the pH, so in the end we made... a derivative of a sugar, one of the sugars of a nuclear side close to the scissile bond and that stopped it. It just made a mutant basically and it didn't change the structured crystals, were exactly the same. So we solved the structure, it was the first all RNA ribozyme to be solved and then for a little while we followed up its mode of action, it seemed to require magnesium. Magnesium required for folding up and we then pursued it, we postulated a mode of action, magnesium was fairly close to the scissile bond but when you have the following enzymatic reactions it's very hard to prove anything unless you can capture intermediate in the reaction. So what we did was, well there was fast freezings going around all over... was using electron microscopy, it was used in fast freezing of crystals in the liquid nitrogen so we fast froze the... tried to capture the reaction, so we found... changed forms on, when you added magnesium but before that we tried to... it was fast freezing with a low temperature, we tried to slow down the reaction so we could catch the intermediate, it's enzymology you see, but this time with RNA, molecular enzymology rather than protein. So we used the sort of tricks that enzymologists had used and we were able to produce a reasonable model of how magnesium brought in the hydroxyl group, magnesium can also bring a hydroxyl group to sort of lend us but magnesium requires a different pH. So we did a lot of studies on the pH control because magnesium has a pK of nine rather than seven, as lead does. So we proposed a mode of action which involved making a cyclic phosphate penta-coordinate in media so it's quite interesting enzymology really and I know enough chemistry. Well I knew about the extra ribonuclease and so on. Which acts on... so we followed that. So we proposed a mode of action and then Bill Scott... by this time I'd got very much involved with zinc fingers which I'll talk about later and Bill Scott and I had a problem, I think he would like to have stayed on in the lab, I had only one position, one post and I decided to concentrate on this zinc fingers because the end Schugman that I'll mention later was around at the time. But Bill Scott had built up a reputation, he had no difficulty getting jobs in the States, he'd married an English girl and so I think he thought he might like to stay in England. Well he'd like to stay in Edinburgh exactly... he was actually Scot... you know merry... of Scottish origin so he went back. So that was a kind of foray into RNA but it was quite interesting and I think it was quite important in its time.
