Chapter 28 



RNA AS GENETIC MATERIAL 



I 



n previous chapters, the DNA- 

 containing phages were the only 

 viruses discussed in detail. In 

 this chapter, we study another group — vi- 

 ruses that contain no DNA and are entirely, 

 or mainly, ribonucleoprotein in content. 

 Members of this group include many of the 

 smaller viruses that attack animals (causing 

 poliomyelitis, influenza, and encephalitis, for 

 example), many viruses that attack plants 

 (such as the tobacco mosaic and the turnip 

 yellow mosaic viruses), and the small RNA- 

 containing bacteriophages 1 (f2, MS2, R17. 

 and others ) . These phages are all extremely 

 similar, but not identical. They are the 

 same size, shape, and molecular weight; they 

 cross react serologically, having similar coat 

 proteins; all attack only male (Hfr or F+) 

 E. coli. 



The usual host for the influenza virus is 

 the mammalian cell. This virus consists of a 

 helical ribonucleoprotein core surrounded by 

 a lipoprotein membrane. It was shown that 

 the lipids in the envelope of the influenza 

 virus are derived mainly from pre-existing 

 lipids of the host cell, and that the composi- 

 tion of the lipids varies with the strain of the 

 host cell. The outer membrane of the virus 

 is apparently derived from the cell membrane 

 and applied when the virus leaves the cell. 

 After infection by the virus, normal cellular 

 growth continues for several hours. There- 



1 See T. Loeb and N. D. Zinder (1961). J. E. 

 Davis and R. L. Sinsheimer ( 1963), and S. Mitra. 

 M. D. Enger, and P. Kaesberg (1963). 



363 



fore, most of the RNA, protein, and DNA 

 synthesized are normal cellular products and 

 bear little relation to the growth of the virus. 

 Using the drug, actinomycin-D, to inhibit 

 normal cellular RNA synthesis, one can 

 demonstrate a specific synthesis of viral 

 RNA. Moreover, with the closely related 

 Newcastle disease virus, which grows in the 

 cytoplasm, one can show that the new (viral) 

 RNA appears in the cytoplasm and not. as in 

 normal cells, in the nucleus. It is probable, 

 therefore, that the internal viral RNA and 

 protein, as well as other viral antigens, such 

 as the hemagglutinating factors, are made 

 under the direction of the virus inside the 

 cell. 



Several genetically-different, haploid strains 

 of influenza virus have been isolated; for 

 example. SWE (with markers, a c) and 

 MEL (with markers. AC). When a mix- 

 ture of the two strains is used to multiply- 

 infect a chick's egg membranes, the mixed 

 infections give progeny particles which, when 

 tested, yield pure clones not only of the 

 parental genotypes but also of stable recom- 

 binant types {A c or a C). Since other ex- 

 planations can be excluded, the results prove 

 that genetic recombination occurs also be- 

 tween RNA-containing viruses. 2 Genetic re- 

 combination has also been demonstrated for 

 the poliomyelitis virus. Whereas the occur- 

 rence of genetic recombination in influenza 

 may require incorporation of two or more 

 pieces of viral RNA into a single particle, 

 recombinant polio-virus RNA seems to occur 

 in one piece. 3 Consequently, although the 

 details of recombination between RNA vi- 

 ruses are unknown, more than one mecha- 

 nism may be involved. 



No evidence has been obtained for the 

 occurrence of genetic recombination among 

 viruses that attack plants. In the case of the 

 tobacco mosaic virus (TMV), infection is 



- Based upon the work of F. M. Burnet and 



others. 



•See G. K. Hirst (1962). 



