IMPLICATION OF A CIS-ACTING ELEMENT IN THE CYTOPLASMIC ACCUMULATION OF UNSPLICED BORNA-DISEASE VIRUS RNAS

Borna disease virus (BDV), the prototype of a new family within the or der Mononegavirales, is unusual in its nuclear localization for replic ation and transcription and use of RNA splicing for gene expression. T he BDV antigenome contains three transcription units and six major ope n reading frames. Multicistronic RNAs containing two introns are elabo rated from the third transcription unit. Differential splicing of the two introns and cytoplasmic accumulation of the unspliced and partiall y spliced RNA are critical for the balanced expression of the putative matrix protein, glycoprotein, and polymerase. To investigate the mech anisms for cytoplasmic expression of unspliced and partially spliced B DV transcripts, the levels of these transcripts were measured in the c ytoplasm of infected COS-7 cells and noninfected COS-7 cells transfect ed with plasmids containing 2.8-kb cDNA inserts representing either wi ld-type or mutant BDV RNA from the third transcription unit. Analysis of truncation mutations allowed the identification of a cis-acting ele ment present within the 3' end of the BDV 2.8-kb transcript that facil itated the cytoplasmic accumulation of unspliced BDV transcripts throu gh nucleocytoplasmic transport. The nucleocytoplasmic transport activi ty was not dependent on the presence of BDV proteins. Gel-shift assays revealed that the cis-acting element binds specifically to host cytop lasmic and nuclear proteins.