RNA AND CONTROL OF CELLULAR PROCESSES 93 



using long time intervals could prove only tliat the nucleolus and 

 puffs accumulated RNA. The real synthesizing site could still be 

 elsewhere in the nucleus since the newly formed RNA could have 

 been transposed into these structures immediately after formation. 

 It was shown in Neurospora (Zalokar, 1960b) that newly formed 

 proteins leave the site of formation within 15 seconds after synthesis, 

 indicating a possibilitv of fast movement of macromolecules inside 

 the cell. Onlv experiments using time intervals of 15 seconds or less 

 could clearlv indicate the prevalence of newly synthesized proteins 

 in the ribosome fraction. Similarly, RNA may move shortly after its 



TABLE 4-1 

 Incorporation of H" Uridine into RNA of Salivary Gland Cell Nuclei of Drosophila 



Silver grains were counted over total area of nucleus (excluding nucleolus) and 

 nucleolus and were calculated for one-day exposure of the autoradiograph. Averages 

 of ten counts are given. 



formation from the chromosomes into the nucleolus. I tried, there- 

 fore, to use extremelv short time intervals in following the uptake of 

 H^ uridine into RNA of Drosophila salivary glands. With H'^ uridine 

 of high specific activity (640 mc/mM), the highly sensitive Ilford 

 G-5 emulsion and 4 months of exposure time, it was possible to de- 

 tect the incorporation into RNA of salivary glands at 5 to 15 second 

 intervals after injection of the precursor into the larva. Both the 

 chromosome and the nucleolus parts of the nucleus became labeled. 

 Silver grain counts gave an approximation of the uptake rate in both 

 fractions. The values for nucleoli and chromosomes increased at a 

 proportional rate for the first 4 minutes ( Table 4-1 ) . There was no 

 indication of prior accumulation of RNA in either chromosomes or 

 nucleolus, which would be the case if one of them supplied newly 

 formed RNA, or a high molecular precursor for RNA, to the other. 

 It could be concluded that RNA was formed independently in the 

 chromosomes and in the nucleolus, and rough estimates, taking into 



