298 H. H. ROBERTS, R. .1. HRITTKX. AM) H. .1. MCfAKTHV 



rich in protein and aiioliu'r rich in KNA which may he a ribosome 

 jirccursior. The 20S particle also appeared after prolonged storage of 

 ribosonies at 10^ M magnesium. In this circumstance a sufficient quan- 

 tity was obtained to show that the protein/nucleic acid ratio was about 

 40/60 but it is not known whether this degradation product is the same 

 as the particle observed in cell juices (Roberts anrl Duerksen, 1960; 

 Aronson and McCarthy, 1961). 



Another particle a])out 43S plays a !)rominent role as a protein 

 deficient precursor to the 50S ribosome (see Section V). It has also 

 been observed in the juice of cells in which DNA synthesis was prevented 

 by lack of thymine (Rol)erts, 1960) and as a degradation jjiodiict of 50S 

 ribosomes (Elson, 1961). 



Abnormal patterns develop when cells are exposed to antibiotics or 

 analogs. Chloramphenicol causes the accumulation of small particles 

 deficient in protein while preventing the entry of P'*- into the normal 

 l)articles (Pardee ef a/., 1957; Nomura and Watson, 1959; Bolton, 

 1959). 5-Fluorouracil causes the accumulation of SOS and 50S ribosomes 

 which are unable to form 70S particles (Aronson, 1961). 



A part of the complexity of the ribosome pattern may be due to the 

 association of newly synthesized RNA or protein with the particles (see 

 Section V,C and VIII). Such material might facilitate or inhibit associa- 

 tion of the basic units. In most experiments its presence would be 

 undetected and uncontrolled, thus giving rise to unpredictable variations. 



D. GENERALITY OF RIBOSOME PATTERNS 



Because of the wide range of ribosome patterns which can l)e observed 

 in one organism it is difficult to compare the ribosomes from different 

 sources. Those which have been most extensively studied show in 

 common particle groups of 70-lOOS which dissociate in low concentra- 

 tions of magnesium into two groups of 2(>-40S and 40-60S. ISIcCarthy has 

 surveyed the ribosomes derived from a number of microorganisms and 

 found a general similarity of patterns (McCarthy, 1959). Some minor 

 diffcrenc(>s appeared but they cannot lie considered significant without 

 extensive studies of the effects of growth conditions. Also, the magnesium 

 concentration required for stability vai'ies from one organism to another. 



In view of these difficulties of intcrcomparing ribosomes and because 

 most of the studies of ribosome synthesis have been carried out with 

 E. coli, it seems best to describe the process of ribosome synthesis as it 

 has been observed in this organism. We hope that this will prove to be a 

 specific example of a general process. Furthermore, we have omitted any 

 reference to the synthesis of RNA in virus-infected cells as the relation 

 of this process to ribosome synthesis is still obscure. 



