Authentic Borna disease virus transcripts are spliced less efficiently than cDNA-derived viral RNAs

Borna disease virus (BDV) is a non-segmented, negative-strand RNA virus tha t replicates and transcribes its genome in the nucleus of infected cells. I t uses the cellular splicing machinery to generate a set of alternatively s pliced mRNAs from the 2.8 and 7.1 kb primary transcripts, each harbouring t wo introns. To determine whether splicing of these transcripts is regulated by viral factors, the extent of splicing was studied in infected cells and COS-7 cells transiently transfected with plasmids encoding the 2.8 kb RNA of BDV. Unspliced RNA was found to be the most abundant RNA species in infe cted cells, whereas viral transcripts lacking both introns were only found in minute amounts. In sharp contrast, plasmid-derived 2.8 kb RNA was predom inantly intron 1-spliced and double-spliced. Co-expression of the BDV prote ins P, N and X did not influence splicing of plasmid-expressed 2.8 kb RNA. Furthermore, the splicing pattern did not change when the 2.8 kb RNA was ex pressed in BDV-infected cells. Based on these results we speculate that spl icing of authentic BDV transcripts is tightly linked to transcription by th e viral polymerase.