Strain-specific expression of spliced MMTV RNAs containing the superantigen gene

The transmission of milk-borne or exogenous mouse mammary tumor virus (MMTV) requires infection of B cells in the gut-associated lymphoid tissue and expression of the superantigen (Sag) protein at the B-cell surface. Presentation of Sag at the B-cell surface is required for the transmission of MMTV to T cells and subsequent infection of the target mammary gland tissue. Because several different promoters have been reported for MMTV sag mRNA expression, we investigated whether the detection of spliced sag RNAs was dependent upon the cell type infected or the particular MMTV strain examined. In this study, we detected expression of spliced sag RNA from the standard promoter and from an internal U3 promoter in B-cell lines expressing endogenous Mtv-6 by RT-PCR, although expression from the standard promoter appeared to be at least 10-fold higher than that observed from the internal U3 promoter. Sag RNA originating from exogenous C3H MMTV was not observed from either of the U3 promoters in any cell type examined. However, spliced mRNAs containing the exogenous C3H MMTV, endogenous Mtv-8, or endogenous Mtv-17 sag genes could be detected from a previously described promoter in the envelope coding region regardless of the cell type infected. Because sag-specific RNAs can be initiated independently of the LTR promoters, there may be selection for independent control of MMTV sag and structural gene expression.