Structural and functional analysis of long terminal repeats of Suncus murinus mammary tumor virus

A 2.7-kilobase (kb) cDNA sequence complementary to Suncus murinus mammary tumor virus (Sm-MTV) genomic RNA [corrected] was prepared using purified virions produced by the Sm-MT cell line, which had been established from a spontaneous mammary tumor of S. murinus. It was found, by using this cDNA in Southern hybridization experiments, that Sm-MTV was endogenous to this animal and that some 50 copies of endogenous provirus were present per haploid cellular genome. In addition, a proviral Sm-MTV DNA sequence, 9.4 kb long (Sm-P-MTV10), was cloned from a Sm-MT cell genomic library, and its long terminal repeat was found to be 720 base pairs (bp) long, with the U3.R and U5 regions 574 and 146 bp long, respectively. The boundary between U3 and R was not determined with certainty, though in the cDNA, the U3 and R regions were 462 and 105 bp long, respectively. The overall homology between the U3.R regions in the cDNA and Sm-P-MTV was 75%. These two DNAs differed in such transcription regulatory signals as CCAAT and TATAA, the first being missing from the cDNA. Nevertheless, chloramphenicol acetyltransferase assays showed that the long terminal repeats of the cDNA and the Sm-P-MTV were transcriptionally active but not steroid hormone responsive. Like Mason-Pfizer monkey virus, Sm-MTV used tRNA(1,2Lys) as a primer for reverse transcription. In addition, the immunosuppressive peptide sequence common to many retroviruses was found in the env region of Sm-MTV. In these two points, Sm-MTV differed from mouse MTV.