This was the construct which was transfected Into NIH 3T3 TK“ cells to make the 
amphotropic packaging line PA-12. 
pPAM Mo-MuLV is a replication-competent type C murine retrovirus which is 
able to Infect mouse and some rat cells and can grow to high titers. It 
produces either T cell leukemias or lymphomas and paralysis when injected into 
newborn mice (Gross. 1970: Gardner. 1978). Naturally occurring murine leukemia 
viruses have been classified into three groups, primarily on the basis of their 
host range. Ecotropic viruses are able to infect only murine and a few other 
rodent cells, xenotroplc viruses are able to infect primarily cells of other 
species (Levy. 1973). and amphotropic viruses have the capacity to infect both 
murine and heterologous cells of many species (Hartley and Rowe. 1976; Rasheed 
et al.. 1976). Mo-MuLV with ecotropic and xenotropic host ranges have been 
Isolated. Biologic and serologic data suggest that viral host range is 
determined primarily by the viral envelope glycoprotein gp70 (Chattopadhyay et 
al.. 1981). 
The Mo-MuLV genome contains 8332 nucleotides which encode gag , pol and 
env . the trans acting proteins Involved In viral encapsldatlon and assembly, 
reverse transcription of the viral genome and integration, and viral budding 
and entrance Into the cell, respectively (Shlnnlck et al.. 1981) (Fig. 1). The 
complete nucleotide sequence of pMLV-1, a Mo-MuLV clone has been determined 
(Shlnnlck et al., 1981) (Fig 2). To make PMLV-1. the unintegrated circular DNA 
form of a Mo-MuLV was cleaved at its unique Hind III site and was cloned into 
bacteriophage lambda Charon 21A (Berns et al.. 1980). This 8.35 kb circularly 
permuted Integrant was subsequently subcloned into the Hind III site of pBR322 
(Miller and Verma, 1984) (Fig. 3) . pMLV-1 is not biologically active, however, 
and could not be used in developing the packaging system. The defect in pMLV-1 
was initially localized to the 5' two-thirds of the genome, spanning the gag 
Recombinant DNA Research, Volume 12 
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