env-Related leukemogenic genes (gp55 genes) of two closely related polycythemic strains of Friend spleen focus-forming virus possess different recombination points with an endogenous mink cell focus-forming virus env gene.

The unintegrated circular provival DNA of the replication-competent, helper Friend murine leukemia virus (F-MuLV) in a polycythemic strain of Friend leukemia virus complex (FLV, K-1 strain) was molecularly cloned after insertion into the EcoRI site of the plasmid pBR322. The nucleotide sequence of its env gene has been determined. Sequences of this F-MuLV env gene, and the previously reported env-related gene (gp55 gene) of Friend spleen focus-forming virus (SFFV) (K-1 strain) were compared to that of the same gene of the other polycythemic SFFV isolate, SFFV (502), recently reported by Clark and Mak. The gp55 genes of the two SFFV isolates are almost identical to each other and have several common characteristic structures. However, the two gp55 genes are different in that the gp55 gene of SFFV (K-1) has a longer nonecotropic MuLV env sequence than that of SFFV (502). This finding may suggest that each SFFV isolate has been generated independently by recombination between a putative defective variant of F-MuLV and an endogenous mink cell focus-forming virus env sequence.