Borna disease virus (BDV) is a nonsegmented negative-strand RNA virus that 
belongs to the Mononegavirales. Unlike other animal viruses of this order, BDV 
replicates and transcribes in the nucleus of infected cells. Previous studies 
have shown that BDV uses RNA splicing machinery for its mRNA expression. In the 
present study, we identified spliced RNAs that use an alternative 3' splice 
site, SA3, in BDV-infected cell lines as well as infected animal brain cells. 
Transient transfection analysis of cDNA clones of BDV RNA revealed that although 
SA3 is a favorable splice site in mammalian cells, utilization of SA3 is 
negatively regulated in infected cells. This negative splicing activity of the 
SA3 site is regulated by a putative cis-acting region, the exon splicing 
suppressor (ESS), within the polymerase exon of BDV. The BDV ESS contains 
similar motifs to other known ESSs present in viral and cellular genes. 
Furthermore, our results indicated that a functional polyadenylation signal just 
upstream of the BDV ESS is also involved in the regulation of alternative 
splicing of BDV. These observations represent the first documentation of complex 
RNA splicing in animal RNA viruses and also provide new insight into the 
mechanism of regulation of alternative splicing in animal viruses.