VII. SYNTHESIS OF RNA AND RIBOSOMES 347 



polymerization of ribonucleotides into a replica of the DNA (Hurwitz 

 et al., 1960; Weiss and Nakamoto, 1961). Finally, the specific radioac- 

 tivity of the RNA found in the first jx^llct (which contains DNA as well 

 as membranes, walls, and unbroken cells) is often unusually high. Thus 

 there are numerous indications that some of the newly formed RNA is 

 synthesized in association with and under the direction of DNA. 



F. DNA-RNA HYBRIDS 



Another indication of the direct participation of DNA in the syn- 

 thesis of RNA is provided by the DNA-RNA hybrids (Hall and Spiegel- 

 man; Spiegelman et al., 1961). These complexes form when RNA and the 

 corresponding DNA are heated and slowly cooled. No hybrids are 

 formed unless homologous DNA and RNA are used, demonstrating that 

 complementarity in sequence and not just similarity in composition is 

 required. Such a condition does not seem possible unless the RNA is 

 formed as a direct copy of the DNA. 



G. SELF-REPLIC.\TION OF RIBOSOMES 



By far the greater part of the ribosomal RNA has no obvious relation- 

 ship in its composition to that of the DNA. The composition of the 

 ribosomal RNA is relatively constant in strains showing a wide variation 

 in the GC/AT ratio of DNA (see Section II, C). It is difficult to visualize 

 how RNA originally DNA-like in composition could be so altered to 

 produce a constant purine-rich composition. It is possible, however, that 

 a limited region of the chromosome, which is similar in many bacteria 

 is particularly active in directing the synthesis of RNA. Such RNA 

 might also be stabilized by the prompt addition of basic proteins. If this 

 were the case a single process could be responsible for the synthesis of 

 both D-RNA and R-RNA. Both types would be copies of DNA and the 

 differences would be attributed to rates of synthesis and stability. 



The alternative view, that the bulk of the ribosomal RNA is formed 

 by a different process, seems less attractive at present. An enzyme has 

 been found which carries out the appropriate reaction, the polymeriza- 

 tion of ribonucleotides under the direction of an RNA primer (Reddi, 

 1961). 



When cells recover from a period of magnesium depletion, during 

 which they lose their complement of ribosomes, the ribosomes increase 

 exponentially. This behavior might be taken to mean that, in the 

 absence of other limiting factors, ribosome synthesis is autocatalytic. 

 There are, however, alternative interpretations. 



At present, the self-replication of ribosomes remains a possibility. If, 

 however, it can be established that all ribosomes contain a component 



