The open reading frame 5A of foxtail mosaic virus is expressed in vivo andis dispensable for systemic infection

Infectious transcripts were successfully derived from full-length cDNA clon es of foxtail mosaic potexvirus (FoMV). Full-length clones were constructed by RT-PCR whereby 5' and 3' genomic segments of 2.7 and 3.4 kb, respective ly, were ligated into Bluescript II KS. The in vitro RNA transcripts were i nfectious to moncotyledonous (barley) and dicotyledonous (Chenopodium amara nticolor) plant species. Individual mutation studies on clones of each of t he five major ORFs confirmed predicted gene function for the polymerase, TG B (triple gene block), and coat protein (CP) genes. Protoplast studies on e xpression of a unique open reading frame, ORF 5A, which initiates 143 nts u pstream of the CP before it "reads through" the CP, revealed that the 5A pr otein was produced in vivo. Mutation analysis of the 5A ORF indicated, howe ver, that it was not required for either replication or for productive infe ction of plants. However, the nucleic acid sequences encoding the extended CP segment were shown to be important for CP expression. Additional mutatio ns in 5A had no effect on FoMV replication in protoplasts but rendered the virus noninfectious to plants. A correlation with diminished CP production from both mutant clones implies that synthesis of subgenomic CP mRNA was co mpromised, and this limited systemic infection.