Tagging the genome of the murine leukemia retrovirus SL3-3 by a bacterial lac operator sequence

The bacterial lactose operator (lacO) was introduced into the PstI site of the long terminal repeat of the SL3-3 murine leukemia virus, generating a virus, SL3-3lacO, that can replicate in NIH3T3 cell cultures. DNA sequences harboring the lacO sequence might be recovered by molecular cloning in Escherichia coli lac+ lacZ+ using bacteriophage lambda or plasmid vectors. The high copy numbers of the lacO sequence titrate out the lac repressor, leading to the induction of the lac operon in the host. We show here that the lacO and the proviral sequences are carried stably together in the genomes of SL3-3lacO-infected cell cultures and in viral particles. This system is designed to facilitate studies on the provirus and the site of viral integration.