Variants of type-C retroviruses from DBA/2 mice: Protein-structural and biological properties

Ecotropic murine leukemia viruses isolated from normal and carcinogen-treated DBA/2 mice can be classified into three main groups that differ in structure and biology. Two groups, called E_a and E_b, consist of N-tropic viruses related to the standard endogenous ecotropic virus of AKR mice. E_a viruses replicate with reduced efficiency in cell lines derived from C3H mice, while E_b viruses essentially replicate normally in these cells. As elsewhere reported, E_a viruses appear apathogenic in C3H mice, while E_b viruses cause a moderate incidence of late leukemias. The biological differences are associated with modulations of the fine structure of the gag gene-encoded proteins.A third group of viruses, called E_c, is clearly more diverged. They differ extensively from E_a and E_b viruses in the products of the gag and env gene, and are related to Rauscher leukemia virus. E_c viruses are NB-ecotropic; they replicate efficiently in all mouse cells tested, and induce leukemias in C3H mice with shorter latency periods than E_b viruses.Since published nucleic acid hybridization data indicate that DBA/2 mice only carry one ecotropic provirus, we assume that the DBA/2 viruses represent a developmental series of variants evolving during the life of the animals.