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DNA Binding and Unwinding Activities Associated with Intracytoplasmic A Particles Isolated from Mouse Mammary Tumours

¹ Laboratory of Molecular Biology, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20205
² Viral Oncology Program, Frederick Cancer Research Center, Frederick, Maryland 21701, U.S.A.

Intracytoplasmic A particles (CAP), previously identified as probable cytoplasmic nucleocapsid precursor structures to mouse mammary tumour virus (MMTV), possess both DNA binding and DNA unwinding activities. CAP proteins bind to both single-stranded (ss)- and double-stranded (ds)DNA, with the ssDNA slightly preferred. This activity was linear over a 30-fold concentration of A particle protein and was not affected by NaCl concentrations as high as 0.6 M or pH changes over a wide range. DNA binding by CAP proteins was sensitive to heat or addition of sodium dodecyl sulphate (SDS) and was neutralized by pre-incubation of CAP with anti-MMTV p14, but not by anti-MMTV p27, p10 or anti-mouse casein. Incubation of CAP with dsDNA led to unwinding of the double helix as measured by its increased sensitivity to S₁ nuclease digestion. This activity was also linear over a several-fold concentration of A particle protein and was heat labile. It was not inhibited by pre-incubation of CAP with either anti-MMTV p14 or with anti-MMTV, anti-MMTV p27 and anti-MMTV p10. DNA unwinding was inhibited by anti-A particle antiserum and to a lesser extent by anti-CAP p20-18.

Received 30 November 1979; accepted 19 February 1980.