Specific sequences of the env gene determine the host range of two XC-negative viruses of the Rauscher virus complex.

Two viruses which do not give rise to XC plaques in the standard XC assay (XC-negative) have been isolated from the Rauscher virus (RV) complex. These viruses differ in their host range. One, R-MCF-1, is dualtropic and will therefore infect both murine and non-murine cells. However, unlike other mink cell focus-inducing (MCF) viruses, it cannot infect NIH 3T3 cells. The other, R-XC-, is ecotropic. It will infect murine cells, including NIH 3T3 cells, but does not infect mink lung cells. Analysis of hybrid viruses, in which homologous regions of the genomes of R-MCF-1 and R-XC- virus were exchanged, indicated that the NH2-terminal portion of the gp70 is responsible for the particular host ranges of these viruses. The nucleotide sequence of the env gene of R-XC- virus was therefore determined and compared with the known env sequences of ecotropic MLVs and dualtropic MCF viruses of the Rauscher and Friend virus complexes. R-XC- virus was found to be a recombinant virus. Its env gene contained sequences derived from an endogenous env gene which were closely related to those of the MCF viruses but differed from any previously described sequences. The particular properties of R-MCF-1 and R-XC- virus suggest that the two viruses arose by recombination between R-MLV and two endogenous env sequences which differ from those of the known MCF viruses. If so, this suggests that the mouse genome contains at least five env sequences which can give rise to MCF-like viruses. In addition, since the host range and interference properties of R-XC- virus are very similar to those of the previously described ecotropic recombinant viruses, it may be that the ecotropic recombinant viruses arose by recombination with the same endogenous env sequences as did R-XC- virus.