BIOCHEMICAL PROPERTIES OF THE ISOLATED NUCLEUS I99 



the presence of thymus DNA. Hall of the nuclei were placed in a nitrogen at- 

 mosphere; the other half remained in air. Alter ^o minutes ol incuhation at 38° 

 the nucleotides were extracted and analyzxd. The results are presented in tahle 6. 

 It is clear that oxygen must be present if DNA is to restore ATP synthesis, and 

 that a DNA supplement under anaerobic conditions has a very small effect indeed. 

 The evidence so far presented indicates that in the intact nucleus deoxyribonu- 

 cleic acid plays a definite role in both amino acid incorporation and ATP syn- 

 thesis. Other synthetic reactions ol the nucleus which are DNA-depcndcnt will 

 now be considered. 



INCORPORATION OF OROTIC ACID-6-C^* AND ADENOSINE-8-c'^ INTO NUCLEAR RNa's 



It has been shown previously that isolated cell nuclei will incorporate orotic 

 acid-6-C^* into RNA pyrimidines, and that both C^^-glycine and adenosine-8-C" 

 will enter into RNA purines (5, 6). Treatment of the nuclei with deoxyribonu- 

 clease impairs its capacity for both orotic acid (6) and adenosine uptake (4). As 

 in the case of amino acid incorporation or ATP synthesis, the addition of poly- 

 nucleotides will restore adenosine-C^* uptake in DNA-depleted nuclei. 



It has also been found that DNA synthesis is inhibited by removal of the de- 

 oxyribonucleic acid. This was first observed by Friedkin and Wood (12), who 

 showed that C^Mhymidine uptake was impaired when thymus nuclei were treated 

 with DNAase. More recent experiments by Sekiguchi and Sibatani have extended 

 these observations to include P^- uptake in both DNA and RNA (23). In their 

 experiments the addition of a DNA supplement also restored P^- incorporation in 

 DNA-depleted nuclei. 



Studies of RNA synthesis in the isolated nucleus, as measured by the incor- 

 poration of both C^*-orotic acid and C^*-adenosine, have indicated a marked 

 heterogeneity of the nuclear ribonucleic acids (3). These experiments will not be 

 described here, but it is of interest that the RNA of the nucleolus is far more 

 active than the soluble and readily extractable RNA of the nucleus. The high 

 metabolic activity of nucleolar RNA in isolated thymus nuclei is thus in accort! 

 with the autoradiographic work just described by Dr. Taylor and also with ob- 

 servations by Dr. Vincent of RNA synthesis in nucleoli isolated from starfish 

 oocytes (25). 



DNA SUBSTITUTION 



From the experiments which have been described testing the synthetic capacities 

 of isolated nuclei, it is clear that DNA plays a central role in the biochemical ac- 

 tivity of the nucleus. Its presence is essential for amino acid incorporation into 

 protein, for adenosine and orotic acid uptake into RNA. and for nuclear ATP 

 production. Protein and RNA synthesis are, of course, dependent upon the nu- 

 clear capacity for ATP synthesis, but the fact remains that all these functions in 



