FIG. 1.

Schematic representation of the MMTV provirus, Sag-reporter proviruses, and regulatory elements for MMTV sag gene expression in B cells. The positions of viral genes (gag, pro, pol, env, and sag), promoters (P₆₉₈, P₁₁₉₆, P₇₂₄₆, and P₈₄₉₈), newly characterized pol gene enhancer (E), and functional regions within the LTRs (U3, R, and U5) are indicated. An additional stimulatory element (hatched box) is located between E and P₇₂₄₆. The Sag-reporter proviruses M^sag17-luc (sag17-luc) and M^sag1-luc (sag1-luc) differ from the MMTV provirus in the 3′ LTRs only. The recombinant 3′ LTRs are depicted below the original LTR. The position of the reporter gene firefly luciferase (luc) in the recombinant constructs is shown. Regulatory elements with demonstrated relevance for sag gene expression are represented as open and hatched boxes.