Envelope gene of the Friend spleen focus-forming virus: deletion and insertions in 3' gp70/p15E-encoding region have resulted in unique features in the primary structure of its protein product.

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Abstract
A nucleotide sequence was determined for the envelope (env) gene of the polycythemia-inducing strain of the acute leukemia-inducing Friend spleen focus-forming virus (SFFV) and from this the amino acid sequence of its gene product, gp52, was deduced. All major elements of the gene were found to be related to genes of other retroviruses that code for functional glycoproteins. Although the carboxyl terminus of gp52 is encoded by sequences highly related to sequences in its putative parent, ecotropic Friend murine leukemia virus, the majority of the protein (69%), including the amino terminus, is encoded by dualtropic virus-like sequences. Nucleotide sequence comparisons suggest that the nonecotropic region may be more closely related to the 5'' substitution in dualtropic mink cell focus-inducing viruses than it is to the 5'' end of xenotropic virus env genes. A large deletion and 2 unique insertions have been located in the env gene of polycythemia-inducing SFFV and may account for some of the unusual structural characteristics, aberrant processing and pathogenic properties of gp52. As a consequence of the delection, amino-terminal gp70 and carboxyl-terminal p15E-encoding sequences are juxtaposed and it appears that translation from the p15E region, 3'' to the deletion, continues in the standard reading frame used by other retroviruses. Insertions of 6 base pairs and 1 base pair at the very 3'' end of the gp52-encoding region results in a SFFV-unique amino acid sequence and a premature termination codon.