Abstract
The transcriptional enhancers of retroviruses that lack an oncogene are important determinants of their oncogenicity. However, no specific cellular transcriptional activator has yet been found to determine the oncogenicity for any of these viruses. The SL3-3 enhancer factor 1 (SEF1) cellular transcriptional activators are expressed preferentially in T lymphocytes. In the SL3-3 murine leukemia virus enhancer, two different sequences can bind SEF1 activators. We show that mutation of the SEF1 binding sites disrupts the disease potential of SL3-3 murine leukemia virus, implying that SEF1 transcriptional activators are required for tumor induction by SL3-3. The SEF1 site mutations did not appear to affect the pathogenicity of SL3-3 by impairment of virus multiplication, but rather by a specific defect in the ability of neoplastic transformation.