Repression of mouse mammary tumor virus transcription by a transcription factor complex. Binding of individual components to separated DNA strands.

Expression of mouse mammary tumor virus in T lymphocytes appears to be required for accession of horizontally transmitted virus to the mammary gland. Further, deletions in the long terminal repeat which relax constraints on viral transcription promote T cell lymphoma. We have identified a polypurine transcriptional repressor element (NRE1) that is deleted from viruses that induce T cell lymphoma. NRE1 binding activity in nuclear extracts proved to be related to a growth inhibitory activity that represses c-myc expression in mature B cells. Mobility shift, DNA footprinting, and UV cross-linking identified three factors that interacted preferentially with double-stranded NRE1 or the separated single strands. NRE1 binding factor (NBF) (80 kDa) bound double and upper strand NRE1, apparently in concert with NRE1 associated factor (NAF) (95 kDa), which interacted directly with DNA only on the upper strand. NRE1 lower strand binding factor (NLF) (50 kDa) cross-linked specifically to lower strand NRE1. On sucrose gradients NBF, NAF, and NLF binding activities cosedimented at 8 S, implying an in vitro association of the 50-, 80-, and 95-kDa factors which precedes DNA binding. Therefore, NRE1 appears to be the site of action of a complex transcriptional repressor comprised of at least three factors that interact differentially with each DNA strand to repress steroid hormone-induced transcription of mouse mammary tumor virus in T cells.