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Thymidine kinase positive (TK+) N type cell lines that had been transformed by spleen focus-forming virus were established by transformation with NB tropic Friend virus complex. Thymidine kinase deficient (TK-) cell clones were isolated. Some of these cell clones release 1000-to 100,000-fold reduced amounts of Friend virus complex as compared to the TK+ parental cell clone. TK-clones were grown in medium without BrdUrd. Some of these TK-clones can be induced to release endogenous helper virus and transforming spleen focus-forming virus on reexposure to 10-10-4 M BrdUrd. BrdUrd inhibits the replication of C-type tumor viruses (1), whereas in some instances BrdUrd or IdUrd can induce endogenous C-type virus (2-4). In order to study the action of BrdUrd on virus replication and its role in the induction of endogenous virus we have used Friend virus (FV) infected and transformed DBA-2 mouse spleen cells in culture. The FV complex consists of two components, the lymphatic leukemia helper virus (LLV-F) and the replication-defective erythroid cell-transforming spleen focus-forming virus (SFFV-F) (5, 6). The RNA of the two virus types is presumably different in size and structure (7). We have established various cell lines transformed by the SFFV-F (8, 9) which, after another thymidine analogue, can be phosphorylated to azidothymidine triphosphate. However, the presence of an No group at the 3' terminal of the deoxysugar interferes with esterification, so presumably azidothymidine can only be added terminally to the growing DNA strand. Both azidothymidine as well as BrdUrd can be used to decrease the virus titer of BrdUrd-sensitive erythroleukemic cells. We have isolated several BrdUrd-resistant clones by treatment of sensitive cells with high doses of BrdUrd (11) and have shown that some of these clones have either decreased or barely detectable thymidine kinase (ATP: thymidine 5'-phosphotransferase, EC 2.7.1.75) activity (11). Furthermore, they have a 108-to 104-fold decreased virus titer. The decrease in C-type virus release can also be shown by electron microscopy.A temporary thymidine kinase activation is observed if BrdUrd is added to some virus negative lines. The same cells release large numbers of endogenous C-type and spleen focusforming virus on exposure to BrdUrd but not to azidothymidine. The RNA tumor virus which is induced by BrdUrd has mainly N-type host range properties unlike the original NB type FV complex which has been used for the transformation of our erythroid cell lines. Thymidine kinase induction and virus induction are always correlated. We have independent evidence (Ostertag, Crozier, and Swetly, unpublished)

Spleen focus-forming virus-transformed erythroleukemic cell clones, which have been established by infection of N type mice with NB tropic Friend virus, continue to release biologically active Friend virus of NB host range. Dimethylsulfoxide induces erythroid differentiation and a 10-to 100-fold increase in the release of biologically active Friend virus. The increase of Friend virus release is a function of the differentiating erythroleukemic cell. The induced Friend virus is not the NB tropic Friend virus complex, but shows N host range. The induction of the Friend virus complex is due to simultaneous induction of both spleen focus-forming and endogenous viruses.Mouse erythroleukemia cells in culture offer an opportunity to study the role of the erythroid cell-transforming spleen focus-forming virus (SFFV) during dimethylsulfoxide (Me2-SO)-induced erythroid differentiation of these cells. Differentiation is blocked by viral transformation; the block can be released specifically by aprotonic solvents (1-3) or by steroids (4). There are two viral components in the Friend virus (FV) complex, the SFFV and a helper virus (LLV-F) (5, 6). The LLV-F can be replaced as helper virus by other C type viruses (6, 7) or by an endogenous virus which is inducible by BrdU (8).In this paper we show that Me2SO, which induces erythroid differentiation in culture, also induces parallel release of an endogenous C type virus and SSFV. This induction is correlated in time with the induction of globin mRNA synthesis in the same cell (9). The release of the block of differentiation which is normally exerted by the SFFV, and the induction of endogenous N tropic and transforming virus but not of the original NB tropic helper virus during differentiation, can be used to study the interaction and normal function of both the endogenous virus and SFFV during differentiation and transformation.

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