REPRESSION OF MOUSE MAMMARY-TUMOR VIRUS TRANSCRIPTION BY A TRANSCRIPTION FACTOR COMPLEX - BINDING OF INDIVIDUAL COMPONENTS TO SEPARATED DNASTRANDS

Expression of mouse mammary tumor virus in T lymphocytes appears to be required for accession of horizontally transmitted virus to the mamma ry gland. Further, deletions in the long terminal repeat which relax c onstraints on viral transcription promote T cell lymphoma. We have ide ntified a polypurine transcriptional repressor element (NRE1) that is deleted from viruses that induce T cell lymphoma. NRE1 binding activit y in nuclear extracts proved to be related to a growth inhibitory acti vity 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 up per strand NRE1, apparently in concert with NRE1 associated factor (NA F) (95 kDa), which interacted directly with DNA only on the upper stra nd. NRE1 lower strand binding factor (NLF) (50 kDa) cross-linked speci fically to lower strand NRE1. On sucrose gradients NBF, NAF, and NLF b inding activities cosedimented at 8 S, implying an in vitro associatio n of the 50-, 80-, and 95-kDa factors which precedes DNA binding. Ther efore, NRE1 appears to be the site of action of a complex transcriptio nal repressor comprised of at least three factors that interact differ entially with each DNA strand to repress steroid hormone-induced trans cription of mouse mammary tumor virus in T cells.