STUDIES ON THE INCORPORATION OF ARGININE 307 



observed in the presence of this compound is obscure, ahhough an in- 

 hibitor normally present in the cell and antagonized by nitroarginine could 

 account for an effect like this. 



When comparing the four guanidino derivatives tested in Table I it 

 can be seen that those in which the electronegative character of the guani- 

 dino group has been decreased (nitroarginine and sulphaguanidine), have 

 no inhibitory action on the arginine-activating enzyme. This is in agree- 

 ment with the observation that the enzyme is relatively acidic and eluted 

 from the DEAE cellulose column with 038 m tris-HCl pH 7-4 whereas 

 the threonine-activating enzyme from calf liver was eluted by 0-12 m 

 tris-HCl pH 7-8 during otherwise similar conditions [25]. 



Sharon and Lipman [26] have studied the influence of analogues on the 

 trvptophan-activating enzyme and have compared their in vitro results 

 with in vivo studies on the growth of E. coli [27, 28]. It was found that 

 5-methyltryptophan and 6-methyltryptophan, inhibited both growth and 

 the tryptophan-activating enzyme. At a concentration ratio of analogue : 

 tryptophan of 200 :i the inhibition was 70" o for 5-methyltryptophan and 

 42% for 6-methyltryptophan in the in vitro experiments. In comparison 

 we found at a concentration ratio of 10:1 (analogue: arginine) 47^0 

 inhibition for canavanine and 38" ^ inhibition for streptomycin. The 

 somewhat greater effect of the arginine analogues lends additional sup- 

 port to the suggestion that the in vivo action of these inhibitors are on 

 the level of the activating enzymes. 



THE ARGININE-RNA 



The alkaline liability of the formed arginine-RNA as well as the other 

 characteristics of the reaction are consistent with the general notion of an 

 amino acid-acyl-RNA compound of the type previously described for 

 other amino acids [29, 30]. 



The difference in the amount of arginine- RX A between the wild type 

 and the arg.R~ mutant (see Figs. 3 and 4) has been observed in two 

 independent pairs of RNA preparations. At present we do not know 

 whether this difference is only a quantitative one or also a qualitative one. 

 The physiological significance of the difference in relation to the mechanism 

 of the arginine repression is also not clear. However, the observed 

 difference supports our original notion that repression is linked with RNA 

 metabolism and encourages us to continue studies along these lines. 



