Distribution of mouse mammary tumor virus-related sequences does not correlate with the taxonomic position of their hosts

Mouse mammary tumor viruses with functional superantigen genes are selected during in vivo infection

Mouse mammary tumor virus (MMTV) encodes a superantigen that is important for viral infectivity in vivo. To determine whether superantigen function was required for infection by milk-borne MMTV, we created HYB PRO/Cla transgenic mice. These mice produced a full-length, packaged viral RNA with a frameshift mutation that caused premature termination of the superantigen protein. Young HYB PRO/Cla mice showed no deletion of their cognate V beta 14+ T cells, although they shed virus in their milk. The nontransgenic offspring of the HYB PRO/Cla mice were infected with this virus, since transgene-specific viral transcripts were detected in their mammary glands. Surprisingly, these offspring demonstrated the progressive deletion of V beta 14+ T cells characteristic of exogenous MMTV (C3H) infection. Sequence analysis demonstrated that these newly acquired viruses had reconstituted superantigen open reading frames resulting from recombination between the HYB PRO/Cla and endogenous Mtv-1 proviral RNAs. Thus, there is selection during the infection process for MMTVs with functional superantigen genes.

Long terminal repeats of dwarf hamster endogenous retrovirus are highly diverged and do not maintain efficient transcription

Recently we described a new endogenous proretrovirus of dwarf hamster Phodopus sungorus (MRS-Ps). Its sequence possesses evident homology with the endonuclease domain of the mouse mammary tumor virus pol gene. Here we present nucleotide sequence data on three clones of retroviral long terminal repeats. As many as 15% of substituted, deleted, and inserted base pairs were found while comparing these sequences. Hence, MRS-Ps seems to be rather an old genetic element which originated about 30 million years ago. One LTR is 877 bp long and contains numerous elements that control its transcriptional activity: TATA-box, glucocorticoid responsive element, NF1-binding site, etc. Nevertheless, this LTR does not govern efficient transcription of adjacent genes in a transient expression assay. In addition, we failed to find MRS-specific mRNA in adult, embryonic, and mammary tumor cells.