562 S. E. LURIA 



IV. Multiplication of Animal Viruses 

 A. Myxovirus Group 



The myxovirus group includes the viruses of influenza, Newcastle disease, 

 mumps, and fowl plague. The particles of these viruses contain at least three 

 antigenically distinct fractions : an ENA-protein element, called the S (or G) 

 antigen; a hemagglutinin (HA) element; and a lipid-containing fraction. 

 Extraction with ethyl ether destroys the particles and permits separation of 

 the S and HA fractions from other materials (Hoyle, 1952), some of which 

 cross-react serologically with antigens of the host tissue (Knight, 1946), 



The changes undergone by the virus particles upon initiation of infection 

 are not yet definitely estabhshed (Hoyle, 1957; R. M. Franklm et al., 1957). 

 It is known, however, that the various components appear at different times 

 and in different parts of the cell, the S antigen, first, m or aromid the nucleus, 

 the HA m the cytoplasm (Liu, 1955; Breitenfeld and Schafer, 1957). Infec- 

 tious virus particles appear later than the S and HA elements. Complete 

 particles are never seen within the ceUs, but only at the cell surface (Morgan 

 et al., 1956). All new infectious virus present at any one time in the uifected 

 ceUs is subject to inactivation by external agents, such as antibody (Rubin 

 et al., 1957), and can be released readily from the cell by treatment with a 

 receptor-destroying enzyme. The unescapable conclusion seems to be that 

 the various virus constituents are formed at different sites within the cell 

 and that their assembly and maturation take place as terminal processes at 

 the ceU surface. The assembly process, however specific it may be, permits or 

 even requires the incorporation into the virus particles of certain materials 

 whose antigenic specificity is host-determined. Such a relatively unspecific 

 process of assembly may at least partly be responsible for the genetic 

 complexity of virus particles produced in cells that receive a mixed infection 

 with two related viruses of this group (Burnet, 1955). It also provides op- 

 portunities for transduction like phenomena in these viruses. 



The RNA-containing S element seems the natural candidate for the 

 primary genetic function in these viruses. Isotopic studies on mfluenza and 

 other myvxoviruses suggest a breakdown of the infecting particles at the 

 surface of the uifected ceU and an initiation of growth by multiphcation of 

 S antigen (Hoyle, 1957). This evidence is somewhat beset by technical diffi- 

 culties, due to the relative instability of the influenza virus. Whether the 

 RNA component can initiate infection by itself, and whether it becomes 

 separated from viral protein as part of the initiation of infection, remain 

 subjects for future study. 



B. Other Viruses 



The rather fragmentary observations on the multiplication of animal 

 viruses of other groups, although they add httle to the picture developed in 



