MULTIPLE MODIFICATIONS IN CIS-ELEMENTS OF THE LONG TERMINAL REPEAT OFRETROVIRAL VECTORS LEAD TO INCREASED EXPRESSION AND DECREASED DNA METHYLATION IN EMBRYONIC CARCINOMA-CELLS

Infection by murine retroviruses in embryonic carcinoma (EC) and embry onic stem cells is highly restricted. The transcriptional unit of the Moloney murine leukemic virus (MoMuLV) long terminal repeat (LTR) is i nactive in EC and embryonic stem cells in association,vith increased p roviral methylation. In this study, expression in F9 EC cells was achi eved from novel retroviral vectors containing three modifications in t he MoMuLV-based retroviral vector: presence of the myeloproliferative sarcoma virus LTR, substitution of the primer binding site, and either deletion of a negative control region at the 5' end of the LTR or ins ertion of a demethylating sequence. We conclude that inhibition of exp ression from the MoMuLV LTR in EC cells is mediated through the additi ve effects of multiple cis-acting elements affecting the state of meth ylation of the provirus.