Organization and expression of endogenous virus-like (VL30) DNA sequences in nontransformed and chemically transformed mouse embryo cells in culture.

A cloned mouse DNA fragment containing an endogenous "virus-like" DNA (VL30 DNA) sequence was identified by virtue of its ability to hybridize to the virus-like RNA component of mixed-pseudotype AKR-murine leukemia virus virions, its lack of detectable sequence homology with cloned AKR-murine leukemia virus DNA, and its hybridization to a 5.6 kilobase pair (30S) cellular polyadenylic acid [poly(A)]-containing RNA species. Restriction enzyme mapping of the cloned mouse fragment revealed the presence of a 5- to 6-kilobase pair VL30 DNA segment flanked by non-VL30 segments of approximately 7 and 0.3 kilobase pairs. Southern blot analysis of VL30 DNA sequence organization in the DNA of two nontransformed mouse cell lines (AKR-2B, C3H/10T 1/2) and two chemically transformed derivatives (AKR-MCA, C3H/MCA-58) revealed 15 to 20 bands organized in an apparent strain-specific pattern. Within a given strain, however, no differences were detectable between the nontransformed cells and their chemically transformed counterparts. The expression of VL30 genes in the above cell lines was assayed by hybridization of 32P-labeled poly(A)-containing polysomal RNA to several internal restriction fragments derived from the cloned VL30 DNA sequence. The level of VL30 RNA was enhanced approximately 10-fold in both chemically transformed cell lines as compared to the nontransformed cell lines (under normal growth conditions). In addition, nontransformed AKR-2B cells maintained in the presence of purified epidermal growth factor exhibited similarly enhanced levels of VL30 RNA sequences in polysomal RNA. Since these cells displayed several growth characteristics of transformed cells but, in an epidermal growth factor-dependent and completely reversible fashion, these data suggest that the expression of VL30 genes is not simply incidental to chemically transformed cells but may be related to alterations in growth control.