Subgenomic RNA as a riboregulator: negative regulation of RNA replication by Barley yellow dwarf virus subgenomic RNA 2.

Barley yellow dwarf virus (BYDV) generates three 3'-coterminal subgenomic RNAs (sgRNAs) in infected cells. Translation of BYDV genomic RNA (gRNA) and sgRNA1 is mediated by the BYDV cap-independent translation element (BTE) in the 3' untranslated region. sgRNAs 2 and 3 are unlikely to be mRNAs. We proposed that accumulation of sgRNA2, which contains the BTE in its 5' UTR, regulates BYDV replication by trans-inhibiting translation of the viral polymerase from genomic RNA (gRNA). Here, we tested this hypothesis and found that: (i) co-inoculation of the BTE or sgRNA2 with BYDV RNA inhibits BYDV RNA accumulation in protoplasts; (ii) Brome mosaic virus (BMV), engineered to contain the BTE, trans-inhibits BYDV replication; and (iii) sgRNA2 generated during BYDV infection trans-inhibits both GFP expression from BMV RNA and translation of a non-viral reporter mRNA. We conclude that sgRNA2, via its BTE, functions as a riboregulator to inhibit translation of gRNA. This may make gRNA available as a replicase template and for encapsidation. Thus, BYDV sgRNA2 joins a growing list of trans-acting regulatory RNAs.