Regulation of Gene Action — Molecular Basis in Higher Organisms 



497 



tween the lysine content of a histone and the 

 melting temperature of the DNA in a nucleo- 

 histone. DNA fully complexed with prota- 

 mine (salmine or clupein) melts at the same 

 temperature as pure native DNA. 



Since an appreciable portion of chromo- 

 somal DNA must be used to carry informa- 

 tion for the synthesis of non-histone pro- 

 teins, not every portion of this DNA can 

 be complexed with a specific type of histone. 

 Consequently, a given type of histone must 

 be able to complex with several different 

 sequences of DNA. 



Although the DNA of duck erythrocytes 

 appears to be almost completely complexed 

 with histone, not every cell has all its DNA 

 histone-complexed. In peas, the fraction of 

 the total DNA not complexed with histone 

 is 5% for the developing cotyledon; 20% 

 for the embryo; 30% for the apices. These 

 results suggest that actively-growing and 

 protein-synthesizing cells have more non- 

 complexed DNA than differentiated cells. 

 The stabilizing effect that histones have upon 

 DNA may involve the suppression of DNA 

 as a template, perhaps by inhibiting DNA 

 strand separation. 



Isolated nuclei can synthesize comple- 

 mentary RNA by using the DNA as a tem- 

 plate; much of this synthesized RNA is mes- 

 senger RNA; some of it is transfer RNA. 

 In isolated calf thymus nuclei, arginine-rich 

 histone added to the incubation medium not 

 only reduces the uptake of thymidine into 

 DNA, but strongly inhibits the synthesis of 

 RNA; ia lysine-rich histones are much less 

 inhibiting. Selective removal of histones 

 from isolated nuclei results in a two- to four- 

 fold increase in RNA synthesis; the newly 

 made RNA is probably messenger RNA, 

 although its base composition is different 

 from that of the messenger RNA normally 

 synthesized. These results strongly indi- 

 cate that the use of DNA as template for 



is See V. G. Allfrey, V. C. Littau. and A. E. 

 Mirsky (1963). 



both DNA and RNA synthesis is inhibited 

 by histones, and the release of DNA from 

 histones can lead to the production of 

 hitherto-repressed messenger RNA. 



Some evidence 1! suggests that the histone 

 control of genetic activity in vivo is essen- 

 tially preserved not only in isolated nuclei 

 but in isolated chromatin as well. Chro- 

 matin isolated from pea embryos, 20 per 

 cent of which is not complexed with his- 

 tone, is able to carry out DNA-dependent 

 synthesis of RNA from the four usual ribo- 

 side triphosphates through the action of 

 RNA polymerase; removal of histone in- 

 creases RNA synthesis about 500 per cent. 

 Fully complexed DNA in a reconstituted 

 nucleohistone either does not support DNA- 

 dependent RNA synthesis at all or does to 

 a considerably reduced extent. Finally, it 

 should be noted that DNA fully complexed 

 with arginine-rich protamine is fully active 

 in DNA-dependent RNA synthesis. 



Pea cotyledons synthesize a specific pea 

 seed reserve globulin not produced in other 

 pea plant tissues such as buds or roots. 

 Chromatin isolated from pea cotyledons will, 

 in vitro, produce the messenger RNA used 

 in a ribosomal system to manufacture this 

 globulin. On the other hand, chromatin 

 isolated from pea buds will not lead to 

 synthesis of this protein; removal of histone 

 from pea bud chromatin, however, yields 

 DNA which supports globulin synthesis. 

 Thus, we see that the gene for globulin syn- 

 thesis is normally repressed in the bud by 

 histone. 15 



Histones may affect gene action by an- 

 other mechanism. 1 '' When already-formed 

 histones are acetylated at their ends, they 

 permit complementary RNA synthesis in 



14 From work of J. Bonner, R. C. C. Huang. R. V. 



Gilden and co-workers. 



'•See J. Bonner, R. C. C. Huang, and R. V. 



Gilden (1963). 



"See V. G. Allfrey, R. Faulkner, and A. E. 



Mirsky (1964). 



