396 RICHARD SCHWEET AND JOHN BISHOP 



of prelabeled (C^^-orotic acidi thymus nuclei to unlabeled cytoplasm 

 upon incubation of the two fractions together. The labeled RXA in the 

 cytoplasm was found associated with microsomal and ribosomal frac- 

 tions. It is clear that intact RXA molecules were transferred from 

 nucleus to cytoplasm, but whether they were incorporated into the 

 ribosomes before or after transfer is not clear. These two shreds of 

 evidence might s^uggest a nuclear origin of cytoplasmic ribosomes. 



2. Synthesis of EXA 



Early enucleation studies with Acetabularia and Amoeba (Brachet, 

 1955) indicated that RXA synthesis was impaired in the absence of a 

 nucleus. The more recent work of Prescott (.1960) and Goldstein et al. 

 (1960) indicates that the inhibition of RXA synthesis may be complete 

 in enucleated Amoeba, Acanthamoeba, and human amnion cells. Recent 

 autoradiographic experiments (Perrj' et al., 1961a) are indicative of more 

 specific effects. After inactivation of the nucleolus with an ultraviolet 

 microbeam. incorporation of cytidine into cytoplasmic RXA was severely 

 inhibited, while incorporation into the nucleus (apart from the nucleolus) 

 was inhibited to a lesser extent. In this case. too. however (Periy et al.. 

 1961b) protein synthesis (as measured by H"-lysine incorporation) was 

 inhibited, although to a lesser extent than RX'A synthesis. 



X'uclear transplantation studies in Amoeba (Goldstein and Plant, 

 1955) showed that a transfer of RX'A from nucleus to cytoplasm oc- 

 curred. The work of Schneider ( 1961) with isolated nuclei is in agreement 

 with this. Kinetic studies of RXA synthesis in HeLa cells ( Feinendegen 

 et al., 1961) and hyphae of Xeuros^p&ra crassa (Zalokar. 1960) indicate 

 that by far the greater part of RXA synthesis in intact cells occurs in 

 the nuclei. The elegant studies of Zalokar (1960) using Xeiiros^pora 

 hyphae centrifuged to separate the different elements of cellular organ- 

 ization, irithaut cell rupture, indicate that the RXA transferred from the 

 nucleus to the cytoplasm becomes associated with the reticulum, pre- 

 smnably with the ribosomes. Thus a nuclear origin of ribosomal RX'A 

 is suggested, but by no means proven. The cytoplasmic synthesis of a 

 metabolically stable ribosomal RX'A would presumably pass undetected 

 in these studies. 



Rho and Bonner (1961) have fractionated pea seedling nuclei pre- 

 labeled with RXA precursors, and have concluded that the primary' 

 site of RX'A synthesis is the chromatin. The newly synthesized RX'A 

 was transferred to the nucleolus. Some autoradiographic studies are in 

 agreement, while others are not. Reference to this work will be found 

 in the paper of Rho and Bonner (1961). 



The type of RXA synthesized in these studies is not known. In many 



