and part of the pol gene (Berns et al.. 1980) and was later pinpointed to a two 
base difference between pMLV-1 and pMLV-48 (Miller and Verma. 1984) (Fig. 4). 
pMLV-K. an infectious hybrid between the wel 1 -characterized pMLV-1 as the 
backbone and the infectious pMLV-48. was constructed by replacing the region 
responsible for the inactivity of pMLV-1 with the homologous region in pMLV-48 
(Miller and Verma. 1984) (Fig. 5). 
To provide broad host range specificity for the packaging vector, the 
envelope protein coding region of pMLV-K was replaced by the env gene of an 
amphotropic clone, 4070A(Miller. et al.. 1985). Strain 4070A was isolated from 
a wild mouse embryo from Lake Casitas Squab Farm (Chattopadhyay et al.. 
1978; 1981; Hartley and Rowe 1976) (Fig. 6). 
The main differences between pMLV-K and p4070A are in the eiw region. The 
envelope glycoprotein encoded by env is the primary determinant of host range. 
The amphotropic envelope glycoprotein of p4070A can bind to surface receptors 
of cells from a wide host range of mammalian species. The ga£ region of 
Mo-MuLV determines N and B tropism (DesGroseillers and Jolicoeur. 1983). 
N-tropic cells are able to replicate only in BALB/c cells and B-tropic Mo-MuLV 
are restricted to NIH 3T3 cells. It was undertaken to construct a hybrid virus 
with the broadest host range possible (Miller et al., 1985). The env region of 
p4070A replaced the homologous region in pMLV-K. The hybrid virus was to 
retain the gag region of pMLV-K which conferred it N/B-tropism. ensuring a 
wider host range than of p4070A which is N-tropic. 
The cloning involved the replacement of the Sal I/Cl al fragment of pMLV-K 
with the homogloous Sal I/Cl al fragment of p4070A. subtending the env^ gene and 
part of the pol region, creating pAM (Fig 7). When pAM helper virus was 
generated after transfection into cells, it had the predicted host range: 
infecting both N and B type mouse cells, a property of pMLV-K. and also 
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Recombinant DNA Research, Volume 12 
