A composite enhancer element directing tissue-specific expression of mousemammary tumor virus requires both ubiquitous and tissue-restricted factors

Mouse mammary tumor virus (MMTV) expression is restricted primarily to mamm ary epithelial cells. Sequences responsible for both the mammary-specific e xpression of MMTV and the activation of cellular oncogenes are located with in two enhancer elements at the 5'-end of the long terminal repeat. Whereas the Ban2 enhancer (-1075 to -978) has been well characterized, the mammary -specific enhancer of MMTV from -956 to -862 has only recently been recogni zed as a key determinant of mammary-specific oncogene activation by MMTV. T he present study identifies and characterizes three binding sites located w ithin this element. Transient transfection of deletion and mutation constru cts shows that all three sites contribute to the basal expression of MMTV i n mammary cells. One of the binding activities (footprint I) is restricted to mammary cells, whereas the other two sites bind factors found in both ma mmary and nonmammary cells. The multimerized mammary-specific enhancer of M MTV on its own can enhance a minimal promoter in a mammary-specific fashion . However, the FpI binding site alone cannot mediate this effect. Thus, it is the binding of multiple factors in a combinatorial fashion that mediates the mammary-restricted expression of MMTV.