< 

 o 



Q. 



q: 

 o 

 o 



a: 



1,2 



0.9 



6 



03 



4000 

 3000 



O 



o> 



-E 2000- 



S 

 a. 



o 



1000 



H -ESTRADIOL 



BINDING OF HORMONE 

 TO PURIFIED CHROMATIN 



H^- PROGESTERONE 



H^'-TESTOSTERONE 



H -HYDROCORTISONE 



Fig. 17. 



CHALKLEY: Well, the conclusion is that it 

 appears to be bound to something which is 

 lighter than histones. It's not bound, apparently, 

 to any great extent to histones, as far as we 

 can see. Possibly it's not bound to histone at 

 all. It is bound to something which precipitates 

 at this concentration of CsCl, and our efforts 

 have been to try and isolate it further. 



EPEL: Have you made any estimates of 

 how many molecules of estradiol there are per 

 nucleus? 



CHALKLEY: No, we haven't yet.* 



Well, now I want to think a little about the 

 problems of repression and what we would have 

 to require of any model to account for repres- 

 sion. We have to be able to explain differential 

 gene effect, the problem of epigenetic control 

 and differentiation. How is it that the pea coty- 

 ledon can synthesize globulin, and yet pea buds 

 cannot synthesize any detectable amount of 

 globulin? We have to involve in this model the 

 fact that a substantial volume of histones does 

 not turn over at all in the lifetime of a given 

 DNA molecule. We have to explain the fact that 

 some do turn over. We have to be able to ex- 

 plain induction of enzyme formation occurring 

 at the genetic level. (This will have to account 

 for hormonal induction). We have to demon- 

 strate that if we induce a system and then re- 



TABLE V 

 Hormone Binding by Chromatin 



Treatment 



PfrCent Solubilised 



BANDING OF H^-ESTRADIOL-CONTAINING CHROMATIN 



* Recent studies suggest a value of about 2500 mole- 

 cules of estradiol per nucleus. 



TUBE NUMBER 



Fig. 18. 



move the inducer that we have a reversal to 

 repression. We have to demonstrate a gene 

 specificity. This has always been a tremendous 

 question mark with histone experiments. It has 

 long been a problem to understand how to 

 selectively repress a gene with simple electro- 

 static interactions. 



John Frenster, of the Rockefeller Institute, 

 produced only recently in Nature (15) a model 

 for the repression and specific gene action. 

 What is proposed is that you have the whole 

 genome entirely wrapped up with histones and 

 repressed. He invokes histones as repressors. 

 He then requires a specific derepressor of RNA 

 that finds the section of the genome with which 

 it is base complementary. It associates with 



143 



