ROLE OF NUCLEIC ACIDS 71 



was able to pass from one cell into the next one (Ledoux et ah, 1954; 

 Brachet and Ledoux, 1955). Indeed, injection of the enzyme is not neces- 

 sary; ribonuclease readily enters the cells of amphibian embryos at the 

 morula stage when these are merely immersed in a dilute ribonuclease 

 solution in tap water. This offered an easy way of affecting RNA in vivo 

 and of exploring the function of RNA in protein synthesis. Simple experi- 

 ments that biochemists had never attempted because of their school years 

 prejudices about permeability of cell membranes, now looked sensible and 

 were soon performed ; many types of cells were found to pick up ribonu- 

 clease. Amoeba proteus for instance rapidly takes up ribonuclease from the 

 medium (Brachet, 1954—55; Schumaker, 1958), most probably by a process 

 known as pinocytosis (Lewis, 1931; Holter and Marshall, 1954; Holter, 

 1959), which consists in the engulfment of droplets of medium; the basic 

 proteins contained in the medium penetrate the cell for they are later found 

 in the cytoplasm and even in the nucleus. In a ribonuclease solution, 

 Amoebae lose part of their basophilia (i.e. RNA is partly destroyed) and at 

 the same time their capacity for incorporating labelled amino acids is 

 almost completely abolished ; respiration is not affected and ATP produc- 

 tion continues, an accumulation of labile phosphate esters is even observed 

 (Skreb-Guilcher, 1955). Ribonuclease thus can inhibit protein synthesis 

 under such conditions that the general metabolic processes are main- 

 tained. 



Another type of ribonuclease sensitive system is the onion root. As 

 observed by Kaufmann et al. (1954, 1957), the pancreatic enzyme causes 

 mitotic abnormalities. The absorption of RNase and other proteins by 

 onion root cells has been confirmed by direct experiments with ^H labelled 

 protein (Jensen and McLaren, 1960); labelled RNase was observed in 

 association with the nucleolus. Brachet (1954) showed that incorporation 

 of amino acids and growth of the root are stopped by ribonuclease. An 

 important observation (Brachet, 1954, 1955, 1956) is that protein synthesis 

 is almost completely suppressed by a rather short ribonuclease treatment 

 (e.g. 30 min). Under these conditions, the extent of RNA degradation is 

 very limited; even after 18 hrs action only 20 per cent of the RNA is re- 

 moved. A limited damage to RNA or the destruction of a certain type of 

 RNA only is enough to suppress protein synthesis. A quite similar con- 

 clusion was reached by Yakeyama et al. (1958) who observed a complete 

 inhibition of fibroin synthesis by ribonuclease in isolated silk glands of 

 Bombyx mori under such conditions that only a limited RNA destruction 

 had occurred. Roots which had been treated by RNase for 30 min, and in 

 which protein synthesis was thus almost completely suppressed, were 

 homogenized and fractionated by high speed centrifugation. No significant 

 difference in RNA content was found in any of the sedimentable fractions 

 between RNase treated and control roots. But a 50 per cent decrease in the 



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