50 CONTROL MECHANISMS IN CELLULAR PROCESSES 



tioning in combination with a possible propagation of secondary 

 templates. 



The possible involvement in repression of a propagation-type be- 

 havior of secondary templates wonld be in harmony with the findings 

 that ribosomes can be in a highly dynamic state and, in some sense, 

 apparently are semiautonomous in that they seem either to be self- 

 duplicating or to participate in determining the rate at which they 

 are synthesized ( Roberts, 1960; Kennell and Magasanik, 1960) . Self- 

 duplication of ribosomes would imply that they can direct the syn- 

 thesis of ribonucleic acid, including information-bearing molecular 

 species. Ribonucleic acid synthesis at the secondary-template level 

 may be thought unlikely in view of the above-mentioned results 

 with fungal, protozoan, and algal systems but should perhaps be kept 

 in mind in the case of bacteria. The rate at which functional ribo- 

 somes are formed could depend, in part, on themselves, if the process 

 of protein synthesis includes ( a ) a splitting of ribosomes ( that may 

 or may not involve a concomitant liberation of nascent enzyme ) and 

 (b) reutilization, for functional ribosome formation, of ribosome 

 fragments, presumably in conjunction with information-bearing ribo- 

 nucleic acids produced in the nucleus. If so, it seems reasonable to 

 assume that the ribosome fragments cannot be reutilized indefinitely 

 and that nuclear control extends not only to the production of such 

 information-bearing ribonucleic acids but also, to a degree, to the 

 production of entire functional ribosomes. 



The Primary Template as a Possible Site of Repressor Action. 

 Arguments in favor of repressor action at the deoxyribonucleic acid 

 level would be based mainly on genetic results and on the behavior 

 of the y8-galactosidase system. Among the relevant genetic results 

 are those on coordinate repression of enzymes whose structural genes 

 are closely linked and those on the activity of operator-type genes. 

 However, the repressibility of enzymes of arginine synthesis, whose 

 genes are not all closely linked, reduces the appeal of such genetic 

 arguments. 



For the ;S-galactosidase system, the possibility of repression and 

 induction at the primary-template level suggests itself in view of the 

 close relationship that has been inferred to exist between this system 

 ( in contrast to certain other systems ) and deoxyribonucleic acid. It 

 remains to be seen, though, if this apparent difference between the 

 y8-galactosidase and other systems is fundamental. The difference 



