The Protein Text 101 



141. T. Deutsch: N-terminal amino acids of gliadins from wheal and rye. Ada Physiol. 

 Acad. Sci. Hiwg. 6, 209-224 (1954). 



142. B. S. ScHWEiGERT, B. T. GuTHNECK, J. M. Price, J. A. MiLiER, and E. C. Milier: 

 Amino acid composition of morphological fractions of rat livers and induced liver 

 tumors. Proc. Soc. Exp. Biol. Med. 72, 495-501 (1949). 



143. G. Rost: Zusammensetzung der ribonucleinsaure der reticulozyten. Naturwissenschaften 

 43,499(1956). 



DISCUSSION 



Koch: I should like to comment on the result of some recent tracer experiments that have 

 been conducted in Dr Swick's laboratory at the Argonne National Laboratory (1, 2, 3). 

 What we have tried to do is to ask ourselves something about the total balance of the turnover 

 of RNA, DNA, and protein in the tissue which is most often studied by the biochemist; 

 namely, rat liver. The interesting thing that comes out of this is that when suitable tracer 

 experiments are done, you can make the definite statement that in a single cell DNA is syn- 

 thesized when it is produced and DNA stays as a cell compound until the death of the ceil, 

 whereas on the other hand it is very easy to show that all of the RNA in the cell is turned over, 

 and it is turned over essentially with about the same half-life that all of the proteins are turned 

 over in the ceil ; that is, there are no special classes of proteins that are not turned over, especially 

 classes of RNA that are not turned over in this tissue. 



The immediate conclusion from this is that, inasmuch as the amount of protein is many 

 times more than the amount of RNA, on a molar or other basis, there can be no one-to-one 

 hand-off of this kind. In other words, you cannot take the DNA and make the RNA from it 

 without using it over and over again in a different way than has been suggested here. 



YcAS : While it may be true that there is turnover of RNA in rat liver, I believe, on the basis 

 of work with micro-organisms, that there is no obligatory turnover of RNA associated with 

 protein synthesis. The RNA, which is part of the protein forming mechanism, is a passive 

 template, and apparent coupling or dissociation of protein and RNA turnover is adequately 

 explained, I think, by the assumption that both have common precursors. 



Koch: I would just like to add that in the case of micro-organisms it is fairly clear that the 

 protein turnover does not occur (4). It is also pretty well established that DNA and RNA 

 turnover do not occur in an actively growing culture. So the concept of turnover in the micro- 

 organism is not a relevant one. But what it does mean is that you cannot accept some of the 

 proposals that have been described that inherently require the obligatory breakdown of some- 

 thing (RNA), concomitant to the synthesis of another type of molecule (protein). 



MoROWiTz: I would like to introduce some evidence for an alternative approach to the 

 problem of intersymbol influence. In some work recently published by Sidney Fox (5) analyses 

 are reported on the total protein of soybean, corn, wheat, and rye. These analyses indicate 

 that a very high proportion of the protein molecules have lysine in an N-terminal position and 

 arginine in the next position. This approach to statistical constraints involves an experimental 

 analysis of a population of proteins from a single source as contrasted to Dr Ycas' theoretical 

 analysis of a population of unrelated proteins. 



We have attempted to determine if any constraints are to be found in E. coli protein. The 

 preliminary results indicate that methionine is found in N-terminal positions in a proportion 

 consistent with a chance distribution. Cystine and cysteine in N-terminal positions may show 

 a considerably greater constraint. 



YcAs: I think that the method used by Fox and yourself introduces an obvious source of 

 bias, if what you are trying to do is look for intersymbol correlations. The abundances of 

 different species of protein in a cell are not equal, and more abundant proteins contribute more 

 end groups. You have to examine the proteins one by one, giving the same statistical weight 

 to each. 



A similarity in end groups of proteins from related species indicates not an effect of inter- 

 symbol correlation, but rather descent from a common ancestor. As can be seen from the data 

 I summarized, proteins change only slowly in evolution. 



Branson: There is one question which has been opened up by Dr Gamow's and Dr Ycas' 

 comments; namely, the whole problem of redundancy in protein molecules. The evidence is 

 fairly conclusive, I believe, that so far as the antigenic action of a protein is concerned, the 



