Identification of a novel RNA splicing pattern as a basis of restricted cell tropism of erythrovirus B19

Prior studies on the transcription of erythrovirus B19 have identified a sh ort leader sequence associated with all spliced viral transcripts. While so me variability has been observed in the acceptor for this first intron, stu dies to date in both permissive and nonpermissive cell types have reported a unique splice donor site. In the semipermissive MB-O2 cell line, we have found that splicing of this first intron proceeds almost exclusively via a cryptic CT donor downstream of the previously reported GT donor at nucleoti de 406. The resulting messages for the viral structural proteins and 11-kDa protein are thereby made bicistronic, with the first expressible polypepti de being a 34 amino acid fusion of the NS-1 and 7.5-kDa proteins. The prese nce of an upstream open-reading frame on these messages is likely to block effective translation of the downstream structural protein products. We pro pose this as a significant mechanism in determining B19's tropism on the ba sis of host cell splicing machinery, and present evidence in support of thi s model. Additionally, this is the first report of usage of a noncanonical splice donor in B19, and to our knowledge the first report of a CT-AG splic e in any system. (C) 2000 Academic Press.