HISTONES FROM TOBACCO CELL CHROMATIN i 

 (cell incubation, 5- FDU inhibition) ' 



20 40 



80 100 120 140 



FRACTION NUMBER 



Fig. 11. 



Biosynthesis of hlstones In cultured tobacco cells after inhibition of DNA synthesis with 

 5-FDU. (Fig, 7, Chalkley and Maurer, Proc. Natl. Acad. Sci. U.S. 54, 498, 1965; repro- 

 duced with permission of the National Academy of Sciences.) 



very simple block to DNA replication, we are 

 inhibiting the formation of certain types of his- 

 tones. 



A somewhat allied topic concerns the repli- 

 cation of DNA in the presence of histones. Al- 

 though it has been suggested that histones might 

 repress the function of RNA polymerase it is 

 evident that many cells are quite capable of 

 maintaining the function of DNA polymerase in 

 the presence of this ubiquitous protein. Thus 

 an in vitro study of the effect of DNA poly- 

 merase upon nucleohistone seemed an exciting 

 realm for study. This has been pursued by 

 Dr. S. Schwimmer. Of particular interest was 

 the fate of the histone associated with the tem- 

 plate. How would it distribute itself among the 

 progenv molecules? 



The plan of his experiments was some- 

 what similar to that adopted for the in vitro 

 analysis of RNA synthesis. He isolated nucleo- 

 histone from calf thymus. This was incubated 

 in the standard fashion for DNA synthesis. 

 The products were examined on a free boundary 

 electrophoresis apparatus (8) previously stand- 

 ardized relative to the electrophoretic mobili- 

 ties of DNA and nucleohistone. The results are 



shown in Fig, 12. This shows that some of the 

 radioactive precursor has been incorporated 

 into material with the mobility of deproteinized 

 DNA. In addition some radioactivity is seen 

 in the region with the mobility expected for 

 nucleohistone. An important question was to 

 find out if the newly synthesized DNA had any 

 histone associated with it. This was answered 

 by exploiting the well known resistance of 

 nucleohistone to DNase during a time period 

 in which DNA alone is extensively degraded. 

 Experiments showed that the newly synthesized 

 material (measured in terms of cpm) is 

 readily solubilized by DNase. Thus a rather 

 curious result arises from these in vitro ex- 

 periments, namely that the daughter strands of 

 DNA are not associated with histone. If the 

 parent molecule was in fact in such an associa- 

 tion it is hard to explain this circumstance. 

 So far there has been no resolution of this 

 apparent inconsistency. 



Evidence exists that the steroid hormones 

 exert their primary effects at the genetic level 

 and thus these hormones seem a useful tool 

 with which to examine the molecular mechan- 

 isms of control in higher organisms. Some 



139 



