Strain-Specific Expression of Spliced MMTV RNAs Containing thesuperantigenGene

Abstract

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 MMTVsagmRNA expression, we investigated whether the detection of splicedsagRNAs was dependent upon the cell type infected or the particular MMTV strain examined. In this study, we detected expression of splicedsagRNA from the standard promoter and from an internal U3 promoter in B-cell lines expressing endogenousMtv-6by RT-PCR, although expression from the standard promoter appeared to be at least 10-fold higher than that observed from the internal U3 promoter.SagRNA 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, endogenousMtv-8,or endogenousMtv-17 saggenes could be detected from a previously described promoter in theenvelopecoding region regardless of the cell type infected. Becausesag-specific RNAs can be initiated independently of the LTR promoters, there may be selection for independent control of MMTVsagand structural gene expression.