A reiterated leader sequence is present in polyomavirus late transcripts produced by a transformed rat cell line

In cells transformed by polyomavirus, the viral genome is integrated into the host DNA, and in the absence of excision, viral gene expression is limited to the early region. We report here that the ability of a unique transformed rat cell line, designated SS1A, to produce readily detectable levels of late mRNAs is due to rearrangements of the integrated viral sequences. The structure of the SS1A insertion, resulting from amplification and deletion events, allows for the formation of a primary late transcript that can subsequently be spliced to generate a reiterated leader attached to the body of the late mRNA coding sequences. The presence of transcripts containing such a leader was confirmed by sequencing the 5' end of cDNA copies of late mRNAs isolated from a library constructed with SS1A mRNA. These results suggest that a reiterated leader sequence is necessary to stabilize late mRNA.