CONTROL BY REPRESSION 49 



cently, for instance, Prescott (1960b) reported that enucleated frag- 

 ments of Tetrahijmena pijriformis are able to synthesize protein, as 

 judged bv the incorporation of labeled amino acids into trichloracetic 

 acid-insoluble material, hwt are completeh^ unable to produce ribo- 

 nucleic acid, as judged b\' the lack of incorporation of labeled cyti- 

 dine into acid-insoluble material. Similar results were obtained with 

 an Acanthamoeha species (Prescott, 1960a, 1960c). In the alga, 

 Acetahularia meditcrranea, the synthesis of ribosomal and soluble 

 ribonucleic acid likewise seems to depend on nuclear function; the 

 synthesis of ribonucleic acid associated with a large granule fraction 

 consisting mainly of chloroplasts, however, apparently is more inde- 

 pendent (Naoraef «/., 1960). 



Tliese studies with fungal, protozoan, and algal systems thus pro- 

 vide evidence that in such organisms the nucleus produces the ribo- 

 somal as well as the soluble ribonucleic acids, which would include 

 those ribonucleic acid species presumed to contain the information 

 for enzyme synthesis. It is possible that such information-bearing 

 ribonucleic acid is present in certain metabolically very active ribo- 

 nucleic acid fractions whose base ratio pattern mimics that of the 

 deoxyribonucleic acid of the organism of their origin. Ribonucleic 

 acid fractions of this type have been demonstrated in the studies of 

 Volkin and Astrachan (1957), Astrachan and Volkin (1958), and 

 Volkin et al. (1958) and in the recent extensive investigations of 

 Nomura et al. (1960), all on the subject of the nucleic acid metabo- 

 lism of phage-infected bacteria, during certain stages of infection. 

 Apparentlv similar metabolicallv active ribonucleic acid fractions 

 have been exhibited in short-term tracer incorporation experiments 

 with uninfected bacteria (Astrachan and Fischer, 1960) as well as 

 with yeast (Yeas and Vincent, 1960). 



The Site Problem. In the light of the available knowledge of 

 repression (and induction) in relation to nucleic acid metabolism, 

 the question may now be examined further, whether repressors 

 and inducers act at the level of the primary (deoxyribonucleic 

 acid) template or of the secondary (ribonucleic acid) template 

 (or, conceivably, of both). Action at the primary-template level 

 would presumabh' be tantamount to action in the production of in- 

 formation-bearing elements of secondary templates; action at the 

 secondary-template level might be directed (a) at the functioning 

 of the secondary templates in enzyme synthesis or ( b ) at such f unc- 



