The readthrough domain of pea enation mosaic virus coat protein is not essential for virus stability in the hemolymph of the pea aphid

A fraction of the coat protein (CP) subunits in virions of members of the family Luteoviridae contain a C-terminal extension called the readthrough domain (RTD). The RTD is necessary for persistent aphid transmission, but its role is unknown. It has been reported to be required for virion stability in the hemolymph. Here, we tested whether this was the case for pea enation mosaic virus (PEMV) virions in the pea aphid (Acyrthosiphon pisum) using RNA1 Delta, a natural deletion mutant lacking the middle portion of the RTD ORF, and CP Delta RTD, in which the entire RTD ORF was deleted. In infected plants, RNA1 Delta virions were as abundant and stable as wild-type (WT) virions, while CP Delta RTD virions were unstable. No RTD of any size was translated from artificial subgenomic mRNA of CP Delta RTD or RNA1 Delta in vitro. Thus, only the major CP was present in the mutant virions. Using real-time RT-PCR to detect virion RNA, no significant differences in the concentration or stability of WT and RNA1 Delta virions were detected in the aphid hemolymph at much longer times than are necessary for virus transmission. Thus, the RTD is not necessary for stability of PEMV RNA in the aphid hemolymph, and it must play another role in aphid transmission.