Discussion 319 



period of gro\\i:h. He then suspends the organisms in a fairly concen- 

 trated suspension, usually containing about 10^^ organisms/nil., and 

 incubates them in the presence of saline and varying concentrations 

 of acetyl ethyleneimine. After exposure to concentrations of acetyl 

 ethyleneimine of the order of 1 in 10,000 for 18 hours, the suspension is 

 plated out and he finds that about 90 per cent of the organisms have 

 been killed (this substance is quite markedly bactericidal). A high pro- 

 portion of abnormal colonies is found amongst the survivors; since after 

 18 hours' incubation on the plate there is a mixture of colonies ranging 

 through white, shades of pink, to red for 24 to 48 hours, then eventually 

 the normal full deep red colour is obtained. In other words there is a 

 delayed pigmentation effect, which is also seen but to a much smaller 

 extent, in control cultures. However, Pryce finds that if he takes a 

 higher concentration of acetyl ethyleneimine (say 1-5 per cent) he only 

 gets a survival of about 1 in 10^ or 10^. The survivors produce colourless 

 colonies, verj^ slow growing, and these properties are retained after 

 repeated serial subculture. These organisms fail to ferment many of the 

 sugars that the parent strain ferments quite easily. As a matter of 

 chemical interest, I might mention that every once in a while, a greenish- 

 yellow variant is produced, but which pigments to the normal colour 

 by contact with a gaseous metabolite seemingly produced by cultures of 

 the normal parent strain. A greenish-yellow pigment has been isolated 

 fi'om these variants and we have shown it to be a pyrrole derivative, 

 evidently an intermediate in the biosjmthesis of prodigiosum itself. 

 Prof. Birch at Manchester is culturing this particular variant in quantity 

 and isolating the greenish-yellow pigment with a view to determining 

 its structure. The point I want to make is that there we have a substance 

 that is a carcinogen, which seems to have produced these apparently 

 permanent changes in a micro-organism. 



W ester gaard: To return to these bifunctional compounds, we are not 

 too happy about crosslinking reactions and induction of mutations. 

 What we do study in these Neurospora tests is the back mutation process. 

 We find that bifunctional compounds are exceedingly efficient on one 

 adenine strain, but so also are monofunctional compounds. The correla- 

 tion is by no means striking. 



Alexander: This is interesting, because in a way it can be made to fit 

 in with what I have just said. If we study cytotoxic phenomena and so 

 on, where presumably just the deletion or loss of DNA is sufficient to 

 produce damage, there the bifunctional or ability to crosslink is the 

 key thing, because it removes DNA as a useful material from the cell 

 much better than a monofunctional agent. When one comes to the more 

 subtle, positive mutations, where a rearrangement is postulated to follow 

 as a result of single breaks, then one would not expect that a bifunctional 

 compound would be any more effective than a monofunctional since it 

 would not give rise to any greater number of single breaks. 



Westergaard: We do find in the monofunctional compound that there 

 is a rather striking correlation between the positive charge on the 

 carbonium ion and the mutagenic effect on the adenine strain. This is 

 the only correlation that we have been able to establish so far with the 



