Mapping sequences required for productive replication of beet necrotic yellow vein virus RNA 3.

Of the four genome components of beet necrotic yellow vein virus only RNAs 1 and 2 are essential for viral replication in leaves. We have mapped cis-regulatory elements on RNA 3 by introducing deletions into expressible cDNA clones and inoculating leaves with the altered transcripts along with RNAs 1 and 2. Transcripts carrying internal deletions extending to within 69 residues of the 3' poly(A) tail or to within about 300 residues of the 5' terminus were efficiently amplified and encapsidated in vivo. The 3' terminal cis-essential domain can be folded into a secondary structure which is conserved among all four genomic RNAs and which probably contains the minus-strand promoter. RNA 3 transcripts with 75% of the central core of the sequence deleted or replaced by the beta-glucuronidase (GUS) gene were also viable. GUS activity was detected in infected tissue in the latter case.