15- 



0.05 - 



20 40 60 80 100 120 140 160 180 200 



FRACTION NUMBER 



Fig. 8. 



Biosynthesis of rat liver hlstones. (Fig. 5, Chalkley and Maurer, Proc. Natl. Acad. Sci. 

 U.S. 54, 498, 1965; reproduced with permission of the National Academy of Sciences.) 



happens if we inject C^^ -leucine into a rat and 

 isolate histone from liver chromatin. The first 

 thing 1 would like to point out here is that the 

 column elution pattern of histones has now 

 changed a little. This isn't surprising as slight 

 variations are found from species to species; 

 though the similarities between histones are 

 often more impressive than the differences. 

 However, again there is labeling in in and IV 

 and the run-off peak and no real elevation above 

 background for the remaining histones. We then 

 wished to examine the patterns of synthesis in 

 the plant kingdom. We selected two systems: pea 

 cotyledons and cultured tobacco cells. In order 

 to avoid the problems of concomitant DNA syn- 

 thesis we employed pea cotyledons from which 

 the growing embryonic axis was cut off im- 

 mediately prior to the experiment. The pea 

 cotyledons were incubated in a sterile medium 

 in the presence of antibiotics and C^"* -leucine. 

 The pattern of labeling found in this type of 

 experiment is shown in Fig. 9. Again there is a 

 slightly different optical pattern indicating 

 slightly different histones. However, we see also 

 the same general pattern found before; that is, 



a small amount of label in the run-off peak and 

 in the III and IV peaks. 



We had one more system which we could 

 conveniently investigate. Here we had tobacco 

 cells growing in exponential growth in a chemi- 

 cally defined medium. DNA synthesis continued 

 apace. They were allowed to incorporate C^*- 

 leucine to study the incorporation into all his- 

 tone fractions. The pattern of histone biosyn- 

 thesis is shown in Fig. 10. The next step was 

 to take these cells and treat them with 5-FDU. 

 We knew from the work of Birnstiel and Flamm 

 (7) that in this system within two hours after a 

 treatment with lO'^ M 5-FDU, we would totally 

 inhibit DNA synthesis, without a serious reduc- 

 tion in RNA synthesis. We could now study a 

 system where we had artificially inhibited DNA 

 synthesis. We must bear in mind that the only 

 thing we've done to alter the system is to im- 

 pose a metabolic block to the formation of 

 thymidine. Figure 11 shows the result of this 

 treatment upon histone biosynthesis. There has 

 been a change to the pattern observed in cells 

 in which DNA synthesis was not normally being 

 synthesized. Thus, it appears that by applying a 



137 



